Work on #87

Implement a test suite, including Travis-CI integration

docs/api_exc.rst

@@ -0,0 +1,80 @@
+==========
+Exceptions
+==========
+
+.. currentmodule:: gpiozero
+
+The following exceptions are defined by GPIO Zero. Please note that multiple
+inheritance is heavily used in the exception hierarchy to make testing for
+exceptions easier. For example, to capture any exception generated by GPIO
+Zero's code::
+
+    from gpiozero import *
+
+    led = PWMLED(17)
+    try:
+        led.value = 2
+    except GPIOZeroError:
+        print('A GPIO Zero error occurred')
+
+Since all GPIO Zero's exceptions descend from :exc:`GPIOZeroError`, this will
+work. However, certain specific errors have multiple parents. For example, in
+the case that an out of range value is passed to :attr:`OutputDevice.value` you
+would expect a :exc:`ValueError` to be raised. In fact, a
+:exc:`OutputDeviceBadValue` error will be raised. However, note that this
+descends from both :exc:`GPIOZeroError` (indirectly) and from :exc:`ValueError`
+so you can still do::
+
+    from gpiozero import *
+
+    led = PWMLED(17)
+    try:
+        led.value = 2
+    except ValueError:
+        print('Bad value specified')
+
+
+.. autoexception:: GPIOZeroError
+
+.. autoexception:: CompositeDeviceError
+
+.. autoexception:: GPIODeviceError
+
+.. autoexception:: GPIODeviceClosed
+
+.. autoexception:: GPIOPinInUse
+
+.. autoexception:: GPIOPinMissing
+
+.. autoexception:: GPIOBadQueueLen
+
+.. autoexception:: InputDeviceError
+
+.. autoexception:: OutputDeviceError
+
+.. autoexception:: OutputDeviceBadValue
+
+.. autoexception:: PinError
+
+.. autoexception:: PinFixedFunction
+
+.. autoexception:: PinInvalidFunction
+
+.. autoexception:: PinInvalidState
+
+.. autoexception:: PinInvalidPull
+
+.. autoexception:: PinInvalidEdges
+
+.. autoexception:: PinSetInput
+
+.. autoexception:: PinFixedPull
+
+.. autoexception:: PinEdgeDetectUnsupported
+
+.. autoexception:: PinPWMError
+
+.. autoexception:: PinPWMUnsupported
+
+.. autoexception:: PinPWMFixedValue
+