Change iteration in landmarks to match dictionaries

This has a nice side effect in that now we can do

if 'key' in image.landmarks:
  print 'Success'
Add the del method to landmarks

Now we can delete landmark groups from landmark managers!
A load more tests for landmarks
Allow the deletion of landmark labels as well

If you try leave a point unlabelled, this will throw an
exception!
More tests for landmarks
Refactor out the unlabelled check

Also, fix some docs
Merge pull request #359 from patricksnape/landmark_upgrades

Landmark upgrades