Mangle stacktrace to exclude mocha and browserify frames

Mangle stacktraces to exclude mocha and browserify frames, also make source filenames relative to cwd. This enables nice stacktraces.

To be honest, I don't think this is the right place for this code but it makes everything so nice so I'd just open PR and let the discussion happen. Probably we can include a generic mechanism for excluding stack frames from stack traces in phantomic and let mochify orchestrate that.