Changed strnstr to strcmp wherever possible in response to ticket #30.

Thanks MrNex.

Altered by me:
* Updated to use strstartswith instead of strncmp.
* Cleaned up xp_parser.c with whitespace cleanups.
* Added very basic xp_parser testcase.
* Fixed spelling of occurrence (again).
* Added missing strstartswith (doctype).
* Properly seek past xml prelude.

Closes #30.