Search filters

Should fix #103.

Add ability to filter search results by pubYear and authorId, for example:

`/search?q=physics+pubYear:2012`

`/search?q=physics+authorId:1df3bab1-1dc7-4017-9b3a-960a87e706b1`

Note: The test in cnxarchive.tests.test_scripts_hits_counter.HitsCounterTestCase is failing because the test data is older than a week and we're no longer adding the data to the recent_hit_ranks table.  I haven't found a way to fix it yet...