Fix typo in server.tasks
Clean-up code for checking external librairies availability

- Enhance code to check if Aubio, Yaafe and Vamp host are available
- Corresponding processors should not be shown in processors list
- TimeSide can now be installed without all or any of these librairies
- Tests ands Doctests will passed (for doctest I had to skip some new server sub-modules)
Merge branch 'dev' of github.com:yomguy/TimeSide into dev