Replace ursa with forsake

Ursa hadn't been updated in two years, and is not compatible with node
v0.11. Since there was user interest in having the library support v0.11
(and thus v0.12 in future), switching to forsake which supports both
v0.10 and v0.11 makes sense.
Merge pull request #12 from mal/switch-ursa-for-forsake

Replace ursa with forsake