Issue 194

Fix for #194. Measurement limit for shapefile export: 500 (configurable) Everything above that will result in a HTTP 500 error:
![2014-06-24-tracktoolongexception](https://cloud.githubusercontent.com/assets/3692794/3371429/a0bb950a-fba0-11e3-836e-f1ba5bd1c934.png)