cleanup for v0pCr3w_exec

Hi bwall,

This pull requests references your https://github.com/rapid7/metasploit-framework/pull/1645

It tries to clean your code, mainly:

* Add the Rank, in this case GreatRanking because it doesn't autodetect windows or unix platform. It you add target auto detection you can put it as ExcellentRanking

* Added two targets: windows and unix since there isn't autodetection in the code really

* It's using the datastore TARGET_URI option used by the HttpClient mixin

Feel free to review, ask or discuss any point, test, and once you agree with this, feel free to land it into your repository and the original pull request https://github.com/rapid7/metasploit-framework/pull/1645 will be automatically updated.

Thanks!