Fixes #38. Multi-dc tokens are now balanced.

Previously tokens were distributed to balance for a single
DC, regardless of how many exists. This now splits the token range
for each DC, using an offset to prevent token collision. This will
make NetworkTopologyStrategy work correctly with multi-DC, but
will break Simple, though the docs recommend against mixing Simple
across multi-dc's.
Merge pull request #149 from pcmanus/balance_multi_dc_tokens

Fixes #38. Multi-dc tokens are now balanced.