Search filters

Added the ability to filter a search by using a f parameter in the url.

There is no way (yet) for the user to affect this - but can be tested by editing the url, for example:

    /search?q="Africa"&f=regions:Africa
or:

    /search?q="Africa"&f=initialPublishDateTime:>2014-10-12T00:00:00Z AND regions:Africa
Perhaps call the public facing parameter 'filter' instead of 'f' ?