Added mssql_escalate_executeas module

This module can be used escalate privileges if the IMPERSONATION privilege has been assigned to the user.
In most cases this results in additional data access, but in some cases it can be used to gain sysadmin privileges.

Lab Setup Guide
https://raw.githubusercontent.com/nullbind/Metasploit-Modules/master/mssql_escalate_executeas_lab_setup.txt

Screen Shots
![v1](https://cloud.githubusercontent.com/assets/1179731/4886796/7d147d78-637f-11e4-87d0-f2c9543ff810.PNG)
![v2](https://cloud.githubusercontent.com/assets/1179731/4886799/80186d54-637f-11e4-86b8-596f623700e3.PNG)