Moved non-necessary struct from accel.h to ...c & moving_avg_ticker.h
Cleaned up unused parts of the header files.
Formalized state validation

When I modify the struct's layout in the future, it becomes much easier
Rearranged the accel_state struct to hide implementation.

This completes rearranging all structs to hide impl, so I ticked it
off
Merge pull request #6 from shalecraig/accel_v2_hide_impl

LGTM. Merging accel v2 hide impl