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. 
I thought the convention was to import specific sub-modules of scipy rather than scipy itself:
```python
from scipy import linalg, optimize, interpolate
```
rather than
```python
import scipy as sp
```

However [I see](http://docs.scipy.org/doc/scipy/reference/tutorial/general.html) that I am wrong. I guess this indicates how often I need to import scipy directly in my own code! 

I am happy to change my import statement to 
```python
import sympy as sym
```
this isn't really that much longer :).