Added SQL SERVER 2008 R2 compatibility

Added SQL SERVER 2008 R2 compatibility (preserving MySQL compatibility) above https://github.com/Saragu0/redmine_impasse/tree/patch-1. 

We tested all features with REDMINE 2.3.3 + SQL SERVER 2008 R2, and with SQL SERVER 2012. 

Now this plugin can be used with SQL SERVER 2008 R2 (and versions above).