Support for 5-star-ratings & bugfixes

Hi,

this pull request contains 4 commits. The first adds support for 5-star-ratings. I think this is related to https://github.com/JaredSartin/Better-Music-Beta/issues/11.
The next two commits fix some bugs (I do not know the exact consequences of the "fix for setting of current playing status" bug, but this is obviously a copy&paste-mistake).
The last commit adds german translation. Note that some Strings (e.g. in popup.html) are not externalized, so they still will appear in English.

Best regards,
Sven