Add ivp solver

This is a placeholder PR to start a discussion on incorporating an IVP solver module into `quantecon`. I have re-written all of the docstrings to conform as closely as possible with NumPy conventions. My version of Sublime is running a PEP-8 checker so there shouldn't be any issues there.

## Tests
I have not yet written any tests. This is the next thing on my TODO list. Off the top of my head good candidates for test cases (i.e., examples that could be re-used in the classroom!) would be:

1. Solow model with Cobb-Douglas production
2. Original "signaling" model due to Spence.
3. Lotka-Volterra "Predator-prey" model.

Any other ODEs with good economic content with closed form solutions that I can use as test cases? I am sure I can think of others.

## Coding style
This is the first time that I have shared this code (outside the classroom) so any feedback on style, etc is greatly appreciated. 
It is conventional to use `sp` as an import alias for scipy. Does it make sense to change the sympy one to something else? I usually use `sym`, but that is a bit longer so I understand if you don't want to do this.