Add minLength option

Allow user to specify a minimum query length that will trigger a search. Useful when searching a large database that may return many results for short query strings. Similar to http://api.jqueryui.com/autocomplete/#option-minLength
Merge branch 'master' of git://github.com/nicklangridge/chosen-ajax-addition into nicklangridge-master
v1 implementation of minLength option.
Removing loading gif when backing out before server request fires off request.
Adding minLength to readme.
Updating example.