support for initial function

[jwkvam]

It would be handy to have a function that get's called on initial page load from the client. Here's a sketch of how this can work:

1. Add an emit from index.jsx
2. Add a handler template in server.py
3. Add functionality to Layout class to call a python function on the initial signal.

[jwkvam]

@dswah if you are interested in learning how to hack on bowtie we can try to do this together.

[jwkvam]

#60