adding styling for intermittent and seasonal Waterways (fix #105)

A couple notes: I didn't use a seasonal=yes styling before zoom level 10 because, at such a large scale, you could not display a river as intermittent without drawing too much attention to it. 

As you zoom to 14, intermittent streams are introduced.

Zoom 12:
![push-z12 png](https://f.cloud.github.com/assets/955351/1641041/5be1c4aa-5848-11e3-95ce-5f142506fd70.png)

Another example at zoom 12:
![push-z12-alt](https://f.cloud.github.com/assets/955351/1641042/667c5268-5848-11e3-812e-a02072903cb1.png)

ZOOM 14, intermittent streams are introduced. 
![push-z14-streams png](https://f.cloud.github.com/assets/955351/1641043/6e0658a8-5848-11e3-8fdc-9ce7434ad823.png)

Comparison of streams and River, in Senegal, west of Fatick:
![push-z15-streamsandrivers-senecomaprison png](https://f.cloud.github.com/assets/955351/1641055/b6642a3a-5848-11e3-918e-bec35d49fca3.png)

Z17: [streams)
![push-z17-streams](https://f.cloud.github.com/assets/955351/1641061/db30e416-5848-11e3-82d0-3955d42b2a3b.png)


z18: (Streams)
![push-z18-streams png](https://f.cloud.github.com/assets/955351/1641062/e4e7d082-5848-11e3-873c-8948c933053a.png)

My only concern is how this looks on very small screens, like tablets or cell phones. I have not tested it on very small screens yet.