Fix autoloads so the entire package is not loaded unconditionally

The use of `;;;###autoload` with the `defpowerline` macro caused `powerline.el` to be
loaded unconditionally at startup. This commit prevents that misbehaviour.
Merge pull request #70 from purcell/master

Fix autoloads so the entire package is not loaded unconditionally
Clear cache before desktop save.
Merge pull request #73 from cabincode/master

Clear cache before desktop save.
Add autoload to powerline-selected-window-active

Possibly fixes #72