Add Carlos Perez's payload injection module

See #1201
Improve Carlos' payload injection module - See #1201

Lots of changes, mainly:
* Description update
* Avoid accessing protected methods
* More careful exception & return value handling
Why yes I know it's a module
Check PID
Merge branch 'payload_inject.rb' of https://github.com/wchen-r7/metasploit-framework into wchen-r7-payload_inject.rb
adding sinn3r as author