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.


/home/unix/aramos/documents/bigwig_support/benchmarks/old/100000/profile_stats.txt
/home/unix/aramos/documents/bigwig_support/benchmarks/old2/100000/profile_stats.txt
/home/unix/aramos/documents/bigwig_support/benchmarks/old3/100000/profile_stats.txt
/home/unix/aramos/documents/bigwig_support/benchmarks/new/100000/profile_stats.txt
/home/unix/aramos/documents/bigwig_support/benchmarks/new2/100000/profile_stats.txt
/home/unix/aramos/documents/bigwig_support/benchmarks/new3/100000/profile_stats.txt