Added experimental support for reading bcf files.

[danielecook]

I've added support for bcf files by using bcftools. Supports fetching specific regions and iteration. Includes checking that indices exist, and that bcftools is available.

[sambrightman]

This seems reasonable if it works. It would be good to:

• add tests to prove it works
• fix the merge conflicts with master
• probably remove which function - seems like a lot of code to add for something simple. there'll be an exception anyway if it's not executable.
• PEP8-ify things a bit (e.g. spaces after commas, use and not condition instead of and condition == False)
• maybe use os.path.splitext on the filename further up, to replace multiple endswith comparisons?