Option to not compile BSON regexes. PYTHON-500

Add a 'compile_re' parameter to Collection.find, Collection.find_one, and
json_util.loads. If it's False, regular expressions are encoded as a new class,
Regex, instead of passed to re.compile(). This allows PyMongo to handle regular
expressions that don't compile in Python but are valid in other contexts like
MongoDB queries.
Unused import