New function in node.py

[sati-bodhi]

I wrote a get_nodes_with_heading function in node.py for my own project so that one can easily zoom-in on nodes with a certain heading in the loaded document.

Example Usage:

from orgparse import loads
root = loads("""
* first
** second
*** third
* fourth
""")
root.get_nodes_with_heading("third")[0].heading
# Output: 'third'

Hope the community would find this useful.