We need to move away from pyinstaller because:

• It causes weird behaviour in mitmdump, like failure to exit on keyboard interrupt
• It doesn't support Python3, which we're heading towards, and probably won't any time soon

I propose that we create mitmproxy installer script instead, which installs mitmproxy and all its dependencies in a virtualenv in the user's home directory, and then symlinks the executables in ~/bin.

This isolates us from the rest of the system, and might also form the basis of an installer for other platforms.