Removed pygame dependency, allow using system tor and update over tor if the latter is used.

[tbbler]

This pull request contains several features:

• I have removed the depency for pygame as this feature is none critical. Instead the option is grayed out and a tooltip appears inviting the user to install pygame if he desires the feature.
• Added an option to use the system wide installation of tor. What this does is set the proper environment variables TOR_SKIP_LAUNCH, TOR_SOCKS_HOST, TOR_SOCKS_PORT.
• When the previous feature is enabled (and the recommended python-txsocksx package is installed) allow the user to do updates (and checks) over Tor. This feature however is only available when using a system install of Tor (as the built-in Tor isn't launched when doing updates).

[tbbler]

Note that the tor port defaults to 9050, ideally it would try to read the torrc file to determine is the port has been changed.

Right now setting the tor control port and password is unimplemented. If this were to be added (so that the Tor Button continues to work when using the system install and about:tor doesn't return a false negative) it should be implemented using something like gnome-keyring so that the password isn't stored in the settings file.

As mentioned, the download over tor feature right now requires the use of the system tor installation. For this to be fixed the launcher would need to launch the TBB's built-in tor first.

Nevertheless, I think this is an important feature for people who are using a system install of Tor, but would like the convenience of auto-update and .desktop launcher icons.

[micahflee]

Sorry about being so negligent! This is great work, I'm reviewing it now.

[micahflee]

And I love making pygame optional, and updating over Tor is something I wanted to build but never got around for it. So I'm excited about these features, thanks so much.