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)
You can add that distances are negative for those points that stay on the 'left side' of the segment direction and whose projection lies within the segment edges. For all other points, distance is positive