Added walk() and find() methods

This pull request resolves #138.

* Added `walk()` method to `stix.Entity`
* Added `find()` method to `stix.Entity` which looks for an `Entity` with a particular `id_`.
* Added `examples/ex_walk.py` which demonstrates a possible use of the `walk()` method

I am positive that the `walk()` method could be implemented better :) It'd be nice to have Python 3's `yield from` syntax!