Add support for Pull Request-based workflows

[alexrinass]

Many people are not merging feature branches locally but wait for review and then merge a branch on their preferred git service (or it gets merged by someone else). We would like to support this workflow better in the following way:

• Provide a git flow <topic> submit command that allows users to create a pull request
• Provide a git flow <topic> status command that list the status of feature branches, detecting stale, open, merged, closed statuses for topic branches
• Provide a git flow <topic> cleanup command that allows users to interactively delete obsolete (merged, closed) branches