Add Query.from_string() class method

This wraps around Query.from_any, but is more intuitive and
understandable by its name.

Change-Id: I5321ed071c508976e89a097d52fb77b8360b951d
Reviewed-on: http://review.couchbase.org/44114
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>