import ratelimitter from 99designs

https://github.com/99designs/goodies/tree/master/ratelimiter

exclude the memcache storage; we don't want to depend on the unused
gomemcache package.
integrate leakybucket algorithm
tests must include leakybucket
include License file of original code
update changelog
Merge pull request #29 from ActiveState/leakybucket

Rate limit using leaky bucket algorithm

for https://bugs.activestate.com/show_bug.cgi?id=103143