GitFlow release: Add support for current workflow

Along with major refactoring of pivotal.py, this changeset implements

1) GitFlow release start <version>
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which does following:

  1) Create release/<version> branch on top of develop.
  2) Go through Pivotal Tracker and get all finished stories (those were
     by definition already merged into develop at least once).
  3) Filter away all stories that are already assigned to a release.
  4) Label remaining stories with 'release-<version>'.
  5) Checkout release/<version>.

2) GitFlow release [--version <version>] list_stories
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which:

  1) Goes through Current and picks all stories labeled as being
     released.
  2) If version is specified, drop all the stories not belonging in the
     chosen release.
  2) Print the stories sorted by the release version number.