Makes the measure mark proportional to image spacing

* Added the parameter radius "Add measurement point" pubsub message. This
  parameter is proportional the image spacing.
* Added PROP_MEASURE to constants.py. This constant is used to set the
  proportion the image spacing.

The measure mark is proportional to the image spacing

Loading measures with circlerepresentation proportional to the image spacing

Adding measures in polydata with circlerepresentation proportional to the image spacing

Added PROP_MEASURE to constants.py
Merge pull request #17 from tfmoraes/measure_spacing

Measure spacing