Add timeout-variant of processEvents

libfreenect also provides a freenect_process_timeout function, that only
   blocks for a specific time.

This can be helpful for moving the processing loop into its seperate
thread, that checks regularly an abort criteria defined by a MVar box
after each timeout.

This commit adds the timeout-based usb processing call.
Add getTiltDegrees call

There is a function missing to retrieve the current angle of kinect's
motor. This commit add it to the haskell interface.

For this we introduce a new datatype for freenect_raw_tilt_state*
pointer and use the given libfreenect interface to get it from the
corresponding instance.

It also automatically updates the internal tilt state, where the
error code is not catched. (libfreenect's examples are also not catching
this failure in general).
Merge pull request #8 from ruuns/master

Some small extension :)