KEYS option

Added the option of searching for specific bibtex entries. This is compatible with other search criteria, as it is implemented as an extra filter rule in multisearch().

$_GET['keys'] should contain a json-encoded array of bibtex entries.

If $_GET['assoc_keys'] is set, then there is an extra behavior. The array in $_GET['keys'] is then considered as an associative array "$abbrv=>$bibkey", and the abbrv field of the bib entry is set (calls setAbbrv($abbrv) ). So, when combined with define('ABBRV_TYPE','x-abbrv'), those $abbrv are used for display.

Also added some breaks in multisearch, for lazy evaluation of conjunction.