Moar better search

Bugfix for #303: when user clicks on country and/or checks past events, these settings are passed to search form and reflected in search results.

There was a tiny tiny regex bug in function that updates search url on List events button: after user clicked on country button, country name was updated, but country code was not.

Also, I added support for search query string in GET requests that will be used for #112 - displaying tags as searchable urls. The parameter is named 'q' - which is typically used name for query strings. 

E.g.: http://127.0.0.1:8000/search/?q=bla+bla

@ercchy Code review... please :)