Popen tests

This changeset should help address the problem in [issue #26](https://github.com/KimiNewt/pyshark/issues/26) - the stderr buffer filling up and causing a deadlock.

As discussed in the issue thread, the main ideas of these changes:
* Make the TShark subprocess part of the Capture class
* Add a helper method to the Capture class that cleans up file descriptors and closes the subprocess

There are some miscellaneous fixes to the docstrings and import cleanups as well.