Clean CmdStagerEcho and Add module targets

Hi @mandreko,

This pull request tries to clean https://github.com/rapid7/metasploit-framework/pull/2874 by:

* Cleaning the CmdStagerEcho, and now hopefully it's compatible with all the sercomm module requirements
* Modifying the sercomm_exec module to add the Netgear N150 target
* Modifying the sercomm_exec module to add manual targets, where the user can configure the exploitation with advanced options:

```
          OptEnum.new('PACKFORMAT', [false, "Pack Format to use", 'VVV', ['VVV', 'NNN']]),
          OptString.new('UPLOADPATH', [false, "Remote path to land the payload", "/tmp" ]),
          OptBool.new('NOARGS', [false, "Don't use the echo -en parameters", false ]),
          OptEnum.new('ENCODING', [false, "Payload encoding to use", 'hex', ['hex', 'octal']]),

``` 

Please feel free to check, review, discuss and land once you're comfortable and verify it's not breaking nothing and all your targets are working smoothly still! :) Thanks a lot!