Boolean operators within words #80

@outcoldman I did some research and small changes. Could you please have a look? Maybe you will have some ideas.

Main problem is evaluateExpression method (generation of expression tree takes around 40 ms).
I removed a loop through the expression tree if it has only one node.
I’ve tested on Mac and Windows Chrome and got following results:
Complex perf test: 251 ms
Search engine perf test: 321 ms.

Also:
_.values - underscore function takes a lot of time. So I'm using only title and url fields during "string" search.

@jamiewilson I fixed bug with "git AND" search string and added tests for this case. Please have a look now.