Faster tree-based t-digest.

Improved tree-based t-digest, with the following changes compared to the
previous implementation:
 - the array structure is stored in parallel arrays instead of a tree of
   objects,
 - data is stored both on inner nodes and leaf nodes,
 - data is updated in-place when this would not trigger a reordering
   of the tree (possible by tracking parents).

Simplistic benchmarks seem to suggest this is a bit faster than the current
ArrayDigest implementation for the default compression (100).
Merge pull request #17 from jpountz/faster_tree

Faster tree-based t-digest.