Make TDigest instances able to summarize more than 2B values.

Centroid counts remain tracked as integers, but whenever counts of several
centroids need to be summed up, a long is used instead. This should allow
for summarizing datasets of at least several tens of billions of values.

Close #14
Merge pull request #15 from jpountz/2b_values_support

Make TDigest instances able to summarize more than 2B values.