Dell MFP 3115cn username enumerator

This module enumerates usernames from the color job log on a Dell MFP 3115cn. The usernames are logged for later use such as brute force attacks. 

Below is an example of the options available in the module.
![dell_mfp-details](https://cloud.githubusercontent.com/assets/3900108/4843651/c3e389fe-6034-11e4-908a-5d01e0b92aef.png)

Testing Steps:
Set RHOSTS to a Dell MFP 3115cn
set RPORT to the printers web admin interface
Set SSL to true if needed.
Run the module and see the usernames get collected. 

Below is an example of the module running and it's output. 
![dell-mfp-username](https://cloud.githubusercontent.com/assets/3900108/4843756/ad275fe6-6035-11e4-8b80-6c06189c0074.png)

Let me know what changes I need to make to the module. :)
your pull_usernames method may return false.

```
1.9.3-p547 :003 > false.nil?
 => false
```