[RFR] Escape slash in Util::escapeTerm (fixes #440)

This PR add slash escape in `Elastica\Util::escapeTerm` method. Indeed, since [Elastic 0.90](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html#_reserved_characters), slashes are a reserved character used for regexp.

Still *Work in Progress* as I did not succeed in launching automated tests yet.