Threaded server

This pull request implement a threaded versuion of MockSSH library. 

This features is really valuable in project where unittest that are using ssh server to retrieve information and parse it. I also modify the tests that show this in practice.

Here an example:
```
class MyTestCase(unittest.TestCase):

    @classmethod
    def setUpClass(cls):
        users = {'testadmin': 'x'}
        MockSSH.threadedServer(commands,
                prompt="$",
                interface="localhost",
                port=9999,
                **users)

    @classmethod
    def tearDownClass(cls):
        print "tearDownClass"
        MockSSH.threadedServerStop()
```