New Feature: Burst Detection (WIP)

This pull-request is server side of https://github.com/jubatus/jubatus_core/pull/43.
Please merge this pull request after https://github.com/jubatus/jubatus_core/pull/43 is merged.

IMO, it's better to change this method signature to `bool add_keyword(0: string keyword, 1: double scaling_param, 2: gamma)`, as arguments of all the other methods take keyword as string.

I don't expect this fix to be done in this pull-req; just for remeberance for future improvements.