Don't assume bgzip is on path when tabix is installed.

I encountered a bug where pybedtools gives an error message
about /tmp/something.gz not existing when trying to tabix
a bed file. This turned out to be because bgzip was missing
from the path (but tabix was present). This checks for both
binaries separately, and results in a more helpful error
message.
Allow bgzip path to be set independently of tabix path.

Also, when checking for bgzip, call it with the -h option
so that it doesn't hang waiting for input on stdin.
Added a regression test for bgzip binary missing from path.
Merge pull request #124 from rbeagrie/master

Don't assume bgzip is on path when tabix is installed.