Pivotal tracker integration.

Added busyflow.pivotal (fixed a XMLBuilder-related bug).
Added reviewboard module.
Reused code from original workflow plugin.
Support for finishing chores.
Support for updating review requests (alpha).
Updating existing review when possible.
Moved busyflow inside gitflow.
Started with release modifications.
Updated docs.
Merge branch 'pivotal-support' of github.com:salsita/gitflow into pivotal-support
Minor refactoring.
Improved release summary.
Fixes.
Updated docs.
Updated doc (minor changes).
Nicer feature start output.
Fixed bug #42789217 (check for whitespace or '/' in branch names).
Fixed bug #42789305 (Better error message when starting an unestimated
story.)
Update README.rst

Minor indentation fix.
Update README.rst
Update README.rst
Adding review-request comments to Pivotal Tracker.

When we post a new review/update an existing one, we post a comment to
PT (to the respective story).
Not: User still has to manually publish the review reqquest in RB.