Simple example

This is a simple example so that people can quickly test whether their installation of pandana is working and whether OpenMP is in use. Successful output looks like:

```sh
> python examples/simple_example.py
Generating contraction hierarchies with 4 threads.
Setting CH node vector of size 1498
Setting CH edge vector of size 1702
[info src/contraction_hierarchies/src/libch.cpp:205] Range graph removed 1900 edges of 3404
. 10% . 20% . 30% . 40% . 50% . 60% . 70% . 80% . 90% . 100%
 100%
```