CRC error checking added

Modified code for handling CRC checking, also makes overall code run
faster. Fixed a bug which was causing distortions in playback of some
files.
Small changes and extra CRC code

Small bug in read_next_header when skipping bytes. Small bug in getbits
(though this would be very rarely encountered). Modified one more
section of CRC checking code. Tidied up some formatting of code.
Merge pull request #3 from soiaf/master

CRC handling code added and bug fix.