[#739] Add support for boost parameter in queries
[#739] Document new search parameters
[#739] Pop extras from query parameters before sending them to Solr

Also move them from the root search params dict to 'extras', so they can
be also be used on GET requests, eg:

http://ckan/api/action/package_search?q=dogs&ext_bbox=1.36,41.13,1.38,41.15

gets transformed to:

{
    'q': 'dogs',
    'extras': {
        'ext_bbox': '1.36,41.13,1.38,41.15'
    }
}
[#739] Partially revert 7d1266 as extensions needs 'extras' on after_search
Merge pull request #758 from okfn/739-add-extra-options-to-query

739 add extra options to query