Rx investigation

This pull request fixes the problem explained in this bug: https://github.com/gem/oq-hazardlib/issues/223

We implemented two new methods in the geodetic.py module called 'min_distance_from_segment' and a 'distance_to_semi_arc'. We also added new tests for these methods as well as for the modified version of the 'get_rx_distance' method in openquake.hazardlib.geo.surface.base.py

The results of the build process on CI are available here:
https://ci.openquake.org/job/zdevel_oq-hazardlib/86/

Here is an example of the results provided by the updated version of the 'get_rx_distance' method
![image](https://cloud.githubusercontent.com/assets/318811/3993843/165a8868-2912-11e4-9466-96dc45c23fbb.png)
Con you extend the doc string similarly to what you have done for 'min_distance_from_segment'?