Fix #459 - properly support remote SPI with pigpio

Sorry! Dave's messing around with the pin implementations again.
Hopefully the last time. The pin_factory is now really a factory object
which can be asked to produce individual pins or pin-based interfaces
like SPI (which can be supported properly via pigpio).

gpiozero/pins/spi.py

@@ -0,0 +1,86 @@
+from __future__ import (
+    unicode_literals,
+    print_function,
+    absolute_import,
+    division,
+    )
+str = type('')
+
+
+import operator
+from threading import RLock
+
+from ..devices import Device, SharedMixin
+from ..input_devices import InputDevice
+from ..output_devices import OutputDevice
+
+
+class SPISoftwareBus(SharedMixin, Device):
+    def __init__(self, clock_pin, mosi_pin, miso_pin):
+        self.lock = None
+        self.clock = None
+        self.mosi = None
+        self.miso = None
+        super(SPISoftwareBus, self).__init__()
+        self.lock = RLock()
+        try:
+            self.clock = OutputDevice(clock_pin, active_high=True)
+            if mosi_pin is not None:
+                self.mosi = OutputDevice(mosi_pin)
+            if miso_pin is not None:
+                self.miso = InputDevice(miso_pin)
+        except:
+            self.close()
+            raise
+
+    def close(self):
+        super(SPISoftwareBus, self).close()
+        if self.lock:
+            with self.lock:
+                if self.miso is not None:
+                    self.miso.close()
+                    self.miso = None
+                if self.mosi is not None:
+                    self.mosi.close()
+                    self.mosi = None
+                if self.clock is not None:
+                    self.clock.close()
+                    self.clock = None
+            self.lock = None
+
+    @property
+    def closed(self):
+        return self.lock is None
+
+    @classmethod
+    def _shared_key(cls, clock_pin, mosi_pin, miso_pin):
+        return (clock_pin, mosi_pin, miso_pin)
+
+    def transfer(self, data, clock_phase=False, lsb_first=False, bits_per_word=8):
+        """
+        Writes data (a list of integer words where each word is assumed to have
+        :attr:`bits_per_word` bits or less) to the SPI interface, and reads an
+        equivalent number of words, returning them as a list of integers.
+        """
+        result = []
+        with self.lock:
+            shift = operator.lshift if lsb_first else operator.rshift
+            for write_word in data:
+                mask = 1 if lsb_first else 1 << (bits_per_word - 1)
+                read_word = 0
+                for _ in range(bits_per_word):
+                    if self.mosi is not None:
+                        self.mosi.value = bool(write_word & mask)
+                    self.clock.on()
+                    if self.miso is not None and not clock_phase:
+                        if self.miso.value:
+                            read_word |= mask
+                    self.clock.off()
+                    if self.miso is not None and clock_phase:
+                        if self.miso.value:
+                            read_word |= mask
+                    mask = shift(mask, 1)
+                result.append(read_word)
+        return result
+
+