Make thread_data global and refactor desktop widget

The desktop widget was the only widget tied to thread_data, and
required to be refactored in the same commit.
Refactor weather widget similar to the other widgets
Add autotools autogenerated files to .gitignore

Ref #11
Fix memory issues in desktop widget
Update installation instructions
Send total/elapsed seconds from mpd status
Lock weather unit to celsius when querying yahoo weather

Weather information needs to be converted to other units in the JS theme file.