adds a ps command to vagrant that drops the user into a remote powershell shell

This adds a new vagrant command:
```
vagrant ps
```
Example:
```
C:\dev\vagrant\win8.1x64> vagrant ps
    default: Creating powershell session to 192.168.1.14:5985
    default: Username: vagrant
[192.168.1.14]: PS C:\Users\vagrant\Documents>
```
The command is very similar to the ssh command but drops the user into a remote powershell session. The command is currently only supported on windows hosts with a guest running via the winrm communicator. The command, similar to `vagrant ssh` also supports a `--command` argument that can execute any powershell command over winrm.

Would be great to add @sneal 's repl to add support for linux hosts.

*note:* this PR is a squashed and final version of #4330