Issue 210 bigwig support

New datasource class for bigwig format files (http://genome.ucsc.edu/goldenpath/help/bigWig.html).

This class uses the ngslib python library to fetch continuous scores from a bigwig file.  For queries spanning a genomic range, the median of scores across the range are returned.


Actually this is probably a more accurate per-call estimate:

{wWigIO.getIntervals} per call time: 0.25 milliseconds.