add double quotes around inserted keywords by default

[valearna]

Right now if the user enters more than one word in a single query box (e.g., 'positive' and 'result') the search is performed with the default lucene engine, which searches all documents containing 'positive' OR 'result'. By adding double quotes we want to force lucene to search for the sentence 'positive result' (since the two words are entered in the same box and not in separate ones).