BED parsing not fully compliant with spec

[mdshw5]

Due to the new AcidBio tests in #188 I now can improve support for BED edge cases in the faidx tool. Currently BED3 support is at 3/18 passed good test cases and 15/15 correctly failed bad test cases. The latter are likely caught at the interpreter level as TypeError or ValueError and could be handled in a more explicit manner.

See here for initial test suite results.