Configuration file support, try 2

This is a new pull request because I rebased and cleaned up some of the older commits.

This spiffy new-and-improved version:

* uses `argparse` to parse command line arguments (username, password, host)
* allows loading a JSON configuration file with `-c` flag
* supplies an example configuration file
* supports multiple configured STUN and TURN servers and randomly chooses one each time
* doesn't crash horribly

The `import` line in `svpn_controller.py` has gotten too long to fit on one line (staying under the 79-char limit). For now, there are two lines, one with a bunch of packages and the second with only `random`. My preference is to have a separate `import` for every package. Another option is to put parentheses around the list of package names so that it can wrap across multiple lines.