output shp fields now correctly represented

Must explicitly cast NumPy types back to native python types for network.write_shp to output edges correctly
![screen shot 2014-09-15 at 10 23 44 am](https://cloud.githubusercontent.com/assets/4959802/4272803/4fc7fa40-3ce5-11e4-9bb6-d15ce5f34d4b.png)

(closes issue #31)