Sparse median

Open pull req for sparse median diginorm.

Thus far there is a new c++ function, and a new python script making use of it.

Algorithm is essentially the same; pseudocode:

'''
if sparse_median_count(read) < COV_CUTOFF:
    consume(read)
else:
    continue
'''


TODO:
* [ ] performance testing ipython notebook
* [ ] implement version of normal median without std dev and avg calcs
* [ ] tests!

Other thoughts, @ctb @mr-c?