Imp copy branches

I'm trying to simplify and generalize the copy_branch method.
I would not force the github repo to have a 'master' branch.
You can set whatever branch you want as the default branch of the github repo, by the 'settings' page.
Thus, I would map the lp 7.0 branches to gh 7.0 branches.

Finally, this can be used for every lp branch. 
For instance, using the following branches.yaml
```
projects:
  - github: git@github.com:eLBati/purchase-report.git
    branches:
    - ['lp:~agilebg/purchase-report/7_fix_1331394', 7_fix_1331394]
```
I moved https://code.launchpad.net/~agilebg/purchase-report/7_fix_1331394 to https://github.com/eLBati/purchase-report

PS: the '--projects' arg should now include the owner of the repo: OCA/purchase-report
Oh, you did the change for the push, but it won't work for the master branch:

    git push github refs/remotes/6.1/master:refs/heads/6.1

is fine, but 

    git push github refs/remotes/master/master:refs/heads/master

is not.

In the use case you gave, we don't need a master branch, but we still want a master branch for the migration of the projects.