Fetch alignments from BAM files using a BAI index

[danvk]

Highlights:

• A few jBinary helpers (uint64native, lazyArray) which aren't being extensively used now, but might be in the future.
• A parser for BAI files
• JavaScript to compute index chunks from a BAI file. This will make supporting side sources of index chunks (i.e. .bam.bai.json files) a cinch in the future.
• A VirtualOffset class to represent "virtual offsets" into BAM files. These pack offsets into the compressed BAM file & the inflated block into uint64s. But those are hard to work with in JS, hence a wrapper.
• Methods to map contig index ↔ contig name for BAM files
• Methods to fetch & parse subsets of a BAM file

And, of course, an implementation of the indexed lookup algorithm. I wound up basing my tests & implementation on htsjdk, which is significantly easier to follow & play with than the canonical C version.

[coveralls]

Coverage decreased (-1.01%) to 92.35% when pulling da0fe83 on bai-parser into ddbaf5d on master.

[coveralls]

Coverage decreased (-0.31%) to 93.05% when pulling 8237e92 on bai-parser into ddbaf5d on master.