Metalinks Code

Added cancel checks to more places for faster stopping and less likelihood for errors, bugfix for downloading non-metalink files (default piece size)

sync with new metalink object methods from appupdater

Cancel callback fix

fixed IE proxy grabbing bug for non-windows platforms

some minor bugfixes

added bitrate/pause/cancel capability, based on file download average, this should probably be changed to a moving average in the future

Improved merge script to handle try/except cases and not double import

Checker code now uses threading which should speed up checks significantly, minor bugfixes

Converted code to use metalink objects and expat for XML for better performance and cleaner code

Added support for using Windows proxy settings, feature 1972561.