Support PuttyMachine file upload and download using non-default port

[kelbyers]

This is a fix for issue #369. If a PuttyMachine instance is created to an ssh server that is running on the non-default port, then the upload and download methods fail, because pscp is still using the default port instead of the actual port.

I have added some tests. The tests verify:

• PuttyMachine calls the __init__ of SshMachine with the correct parameters
• PuttyMachine includes the correct port option for pscp with non-default ports
• PuttyMachine.__str__ has the expected message.

The tests can run on any system even without Putty installed. One of the test fails on PyPy, but it is marked with @xfail_on_pypy. It fails on PyPy because the feature of pytest-mock used in that test does not work on PyPy.

[coveralls]

Coverage increased (+0.3%) to 81.554% when pulling cfea104 on kelbyers:master into 4ea9e4c on tomerfiliba:master.

[henryiii]

I'll be getting back to pull requests in a few days; I've been relocating from US to France and still don't have internet access at home.

[henryiii]

Looks great! Thanks