0.1.7 fix

Hello there!

First off, thank you so much for maintaining this amazingly helpful gem!  It has proven it's worth time and time again with my work at @sifteo as well as some of my personal projects.  

That being said, I noticed today that our development environment experienced some regressions due to some recent changes of `query_reviewer`.  More specificly, the changeset between `0.1.6` and `0.1.7`.  

After some debugging, I was able to determine that [this particular commit](https://github.com/nesquena/query_reviewer/commit/352d2db3afe1e88f37264bc50212e0fc6fcd3ec5) introduced a regression where `Thread.current["query_reviewer_enabled"]` was not being set, which prevented queries from being inserted into `Thread.current["queries"]`, as described [here](https://github.com/nesquena/query_reviewer/blob/master/lib/query_reviewer/mysql_adapter_extensions.rb#L80).

Another regression also introduced [not being able to render controller actions](http://stackoverflow.com/questions/14763584/rails-3-1-10-upgrade-results-in-consistent-get-miss) for users that did not set the cookie that allows query_reviewer to work its magic.

This set of commits aims to be the least-amount-of-work to get back to a fully functional state, though for all intents and purposes, it is more-or-less a revert to a previous state.

Thanks!