Added Rank Expressions to CloudSearch Requests

Adding a rank_expressions dictionary to any passed Query object will allow
for non-stored Rank Expressions to be added to the search.

This feature is defined by AWS here:
http://docs.aws.amazon.com/cloudsearch/latest/developerguide/rankexpressionquery.html
Moved rank_expression parameter to the end of passed arguments

This will help to not negatively affect people using positional
arguments (as recommended by @kopertop.)
Merging change to allow rank expressions for search queries