"autobin": determine bin sizes from given BAM(s)

[etal]

This new command would dynamically determine on- and off-target bin sizes given a reference genome FASTA and one or more (?) BAM files corresponding to normal samples -- essentially doing access, target, antitarget and coverage simultaneously and emitting target and antitarget .cnn and perhaps BED files.

Possible implementations:

1. Use a fast estimate of overall coverage depth(s) (e.g. samtools idxstats and the target and optionally access BED files to compute and compare the sizes of sequenced areas) and simply print the recommended average bin sizes to use with target and antitarget.
2. Stream through one or more BAMs using samtools depth or pysam count() or count_coverage(), and emit target and antitarget bin coordinates and coverage depths designed to place approximately a constant number of reads in each bin. Two passes or some backtracking might be needed to avoid "remainder" reads/bins, unless we can use (1) for hints. Some more tuning options may be necessary.

See also: bcbio/bcbio-nextgen#1582