Try to prevent nil URL from being passed to AFNetworking (results in exception).
Don't expose mutators for state variables in WSDeviceLink.
Rename HTTP methods.
Refer to the enumeration SensorOperationType throughout the project when using sensor operations (was int -- unsafe).
Move device link constants to the device link constants header file.
Comment properties.
Make the sessionID read-only.

This brings up an interesting point: WSDeviceLink seems to be an object that just handles network communications, and so it takes a sessionID and sourceObjectID with every API call.  However, we store the session ID so that we can chain together commands, which breaks this idea.  This needs to change (related to github issue #148).
sourceObjectID doesn't make sense and was being used inconsistently.  Change to deviceID.
Comment API endpoints.
Wasn't setting the NSError object in the delegate, so some error messages weren't appearing.