Added HAR generation as inline script

As promised yesterday, this one goes open source ;)
As long as we have this feature as an inline script, I'm inclined to remove the dependency here. 
I'd suggest that we just try to import harparser and pytz and throw an error instructing the user to install the dependencies (catch for `ImportError`).
In the long term, we probably want to include that in the mitmproxy core, but we first want to introduce a clean extension interface before that's going to happen. As soon as said interface is there, porting the inline script and adding the dependency again should be a breeze.