Add GovCloud region(s)

This removes GovCloud regions by default (ensuring existing code will work unchanged) and adds the following two parameters to enable GovCloud usage regardless (modeled after how the [AWS Tools for Windows PowerShell](http://aws.amazon.com/powershell/ this) :
```
--include_govcloud ("Include 'GovCloud' regions. [default: False]")
--only_govcloud ("Return only 'GovCloud' region(s). [default: False]")
```

:information_source: The boto dependency has been updated to 2.15.0 accordingly.