KEYS option

Added the option of filtering bibtex entries based on a list of specified entries. This is compatible with other search criteria, as it is implemented as an extra filter rule in multisearch(). This is useful for presenting a selection or short-list of bibtex entries (eg, on a homepage, in a research project page, etc...)

$_GET['keys'] should contain a url-encoded json-encoded array of bibtex entries.

With @define(ABBRV_TYPE,'keys-index'), the array in $_GET['keys'] is considered as an associative array "$abbrv=>$bibkey" and $abbrv is used for abbreviation.

Also:
- breaks added in multisearch, for lazy evaluation of conjunction.
- added function to sort entries by undecorated abbrv