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
Remove now unused FileLookup._encode().
Merge pull request #154 from zas/browser_options

Browser options