Include small samples of 2bit/BB/bam files for easier testing

[armish]

We currently depend on external 2bit (for reference) and BB (for genes) files for testing/demoing. Instead, it would be great to have relatively small slices of them in the repository.

These will include the following files that span the range chr17:7500000-7515000

• [ ] a slice of the hg19.2bit file
• [ ] a slice of the ensGene.bb file
• [ ] a small bam files with unpaired reads

[danvk]

one problem is that there's no such thing as a 2bit "slice" — each chromosome always starts at position 1. We could work around this by making a ShiftedTwoBit class that wraps TwoBit, but this might be confusing for our introductory example.

[armish]

ah, I see. That 2bit file will require some more thinking then.