Search functionality for containers

As discussed here: https://groups.google.com/forum/#!topic/construct3/7abbfuCGAUI

I added *search* and *search_all* for searching specific tags inside containers. *search* only returns the first occurrence, *search_all* returns all of them.
This should allow users to easily check their Constructs for specific data.
I've implemented some basic tests, they probably dont cover all of the end-cases so if you see anything thats missing, let me know.

As a side note, it could be interesting adding something similar to XPath for more sophisticated searches as done here: http://docs.python.org/2/library/xml.etree.elementtree.html#elementtree-xpath