Add omnisharp_onsave_codecheck setting

This commit allow to user disable code checking after saving. I think there are a lot people who press super+s very frequently and hate this jumping window with warnings ;) 
User can just add this line to his "sublime-settings":
*"omnisharp_onsave_codecheck": true*
![Screenshot](http://gyazo.halfbusstudio.com/b46b5d.png)