Adding a ps command for entering a remote powershell session

This PR is not quite ready for merging but wanted to add now just to inquire if it would be considered as a new feature. 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.

Currently it is assumed that powershell remoting is enabled on host and guest.

Things to add before merging:
* Prompt the user to enable remoting if it is not already enabled
* Add a -Force option that would do the enabling without prompting
* Raise appropriate errors if user is not enable and does not accept the prompt
* Add a -command option that would remotely execute a command on the guest similar to the same option in `vagrant ssh`. This can use winrm directly and would not need ps remoting enabled.

All of the above TO DOs are straight forward and I will have them included in this branch this week.