Implement process tree feature

This is the implementation of the process tree feature as discussed in #69.

The tree display works for the console frontend only, and should not affect other frontends.
It is enabled by passing the command line switch `--tree`.

Tested on Python 2.7 and 3.4 on Linux (Ubuntu 14.04), and Python 3.3 on Windows XP, with and without the `--hide-kernel-threads` option.