Merge pull request #15 from rojkov/master

Use system nspr for nemo builds
Added Context menu handler from metro browser
Added selection handler from metro browser.
Added clipboard override component in order to avoid bad behaviour coming from old default Qt implementation (expecting UI thread same as content thread)
Provide function which invoke context menu
Disable debug json messaging
Switch back to native json messaging via tabChild interface
Extend dl-start and dl-done messages sent to UI