Merge pull request #12 from coolo/master

  several commits collected
simplify rqlink usage by offering osc staging select LETTER REQUESTS...
Merge pull request #13 from coolo/have_select

simplify rqlink usage by offering osc staging select LETTER REQUESTS...
Remove some superfluous code
Extract StagingApi class from osc-staging
Update documentation about plugin installation
Integrate coolo's changes in StagingApi class
retry rqlink - select is the better interface