Move Tomahawk player URL generation to server.py.
Replace has_key() with `key in ...`.

`has_key()` has been deprecated since at least Python 2.6, and the
AcousticBrainz server requires Python 2.7 anyway.

https://docs.python.org/2.6/library/stdtypes.html?highlight=has_key#dict.has_key
https://docs.python.org/2/library/2to3.html?highlight=has_key#2to3fixer-has_key

Fixed using: `2to3 -f has_key .`
Fix "E713 test for membership should be 'not in'".

https://pep8.readthedocs.org/en/latest/intro.html#error-codes
Remove trailing whitespace.

Using: `find . -not \( -name .git -prune \) -type f -print0 | xargs -0 sed -i -E 's/[[:space:]]*$//'`
Merge pull request #41 from Freso/various

Various code clean-ups