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.

I'm happy to write a regression test for this if you feel it's necessary, but I can't think of a nice clean way of doing it off the top of my head.