Merge pull request #5 from OpenSCAP/oscap_source

Oscap source
Find libbz2 if available.
Open bzip2ed files nativaly.

When their name ends with '.xml.bz2'.
Make sure to never run BZ2_bzRead after eof

Addressing:
OpenSCAP Error: Could not read from bZ2FILE: SEQUENCE_ERROR [bz2.c:77]
Change prototype of bz2_read to avoid a lot of casts.
tests: The very first test for bz2 openner.