RLS: Get easy backport tool from IPython master.

RLS: Add attribution.
RLS: Update gh_api to curren IPython master.
RLS: Allow getting by milestone.
Merge pull request #1136 from jseabold/backport-tool

RLS: Add IPython tools for easier backporting of issues.
MAINT: Add stub for 0.6.0 release notes.
DOC: Update contributing notes.
DOC: Add docs on commit messages to contributing.