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.


test input file: maflite file with 100,547 variants
test bigwig file: wgEncodeCrgMapabilityAlign100mer.bigWig provided by Esther

total Oncotator runtime without bigwig: 67.6 minutes
total Oncotator runtime with bigwig: 71.3 minutes

BigWig.py:18(annotate_mutation) per call time: 0.02 milliseconds