Praedasploit xerox ldap

This PR is the Praedasploit Xerox workcentre 5735 LDAP credential extractor. This module performs an LDAP pass-back-attack against the Xerox printer and captures the LDAP credentials configured on the printer. 

Below is an example of the options for for the module:
![xerox_ldap_options](https://cloud.githubusercontent.com/assets/3900108/4799414/43ed8886-5e1c-11e4-9b87-49a8dd4a013a.png)
 
Testing Steps:
Set NewLDAPServer to the IP address of the system running Metasploit
Set RHOST to the IP address of the Xerox printer
Set RPORT to the XEROX WorkCentre web management interface
Set SRVHOST to the IP address to listen on. This is usually the system running Metasploit 
set SRVPORT to the local port to listen on. This is the port the Xerox printer will connect back on.

Now run the module. The following image is an example of the module running on a test Xerox workcentre 5735 printer. 

![xerox_ldap_capture](https://cloud.githubusercontent.com/assets/3900108/4799872/5a69e2fe-5e20-11e4-8a10-96e759601850.png)

Any questions please let me know. 
@todb-r7 used to reject modules using Nokogiri... but I'm not sure what happened, that's not documented in the wiki. I'll let him decide if this is okay or not.