Fix problem with wrong window ID/winnr after some window operations

Removed autocommands as well. Powerline() function is now responsible both for
displaying powerline and for updating statuslines. No cmdwin events not
triggered workaround nonsense anymore.

Fixes #237
Fixes #236
Fixes #222
Fixes #232
Handle non-existant CWDs gracefully
Fix various minor formatting issues

Closes #242.
Remove obsolete Vundle warning from docs
Fix &stl reset to global value when new file is loaded

Fixes #241
Ref #240
Don't use UUID for window IDs

This fix uses an incremented number instead.

Fixes #180