bench: add a BAM/SAM parsing benchmark

This benchmark is first step towards tackling #123.

To integrate with other benchmark I added a `main.ml` which creates the
`Command.group` which then allows to call the specific benchmarks.

The code is based on @pveber's code sent to the mailing-list.
travis: make benchmarks command return 0
Merge pull request #124 from smondet/bamsam_bench

Add a BAM/SAM parsing benchmark (#123)