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.


The db-dir I used can be found here:  /xchip/cga_home/aramos/oncotator/db_dirs/oncotator_v1_ds_Nov042014
It's the same as the previous corpus but with the wgEncodeCrgMapabilityAlign100mer datasource.