Use a smarter method to extract track number from filename.

When file has no metadata field indicating the track number, Picard
is trying to guess it from filename, but the simple regex wasn't very efficient.

tracknum_from_filename() does a much better job.
A test case was added.
Use an url builder to simplify code.

_url() will ensure correct query building, encoding, query parameters are passed
as a dictionary
Add options to control browser integration (enabled/disabled and listening port)
Rename "Web Proxy" option page to "Network"

Settings for web proxy are still there, but we'll add browser integration
options to this option page too.
Move Browser Integration options to Advanced > Network option page
Add a real life test for track number case extraction
Remove now unused FileLookup._encode().
Merge pull request #154 from zas/browser_options

Browser options
Move constant regexps outside the `tracknum_from_filename()` function
Merge pull request #151 from zas/tracknum

Use a smarter method to extract track number from filename.
Adjust the replaygain plugin to the changes in threading