Added an "elapsed" and "query count" log threshold optional setting and documented the settings in readme

This is a very useful tool to debug django db performance.. but data can get cluttered between tons of small and meaningless requests. this allows the middleware user to selected the query elapsed time and the minimal query count that he wants the middleware to log. Enabling the user to focus on bottlenecks and problems. 

Without explicitly changing the settings the current behavior isn't changed in any way