First draft of psexec client (PSE) class

please let me know what you think. 

usage:
```
import tunnel
form pse import PSE

client = tunnel.get_sshclient(bastion.server,username='bastion.user')
pse_client = PSE(password="P@ssw0rd", host="123.45.67.89", username="Administrator", bastion=client)
pse_client.test_connection()
```

Nico