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
indicies over mask.true_indices in LK, doc fixes
doc changes in affine and homogeneous
remove debug break apart of invertible
Merge pull request #362 from jabooth/smallfixes

Assorted doc fixes
Fixes orthonormalize_against method edge case.
Merge branch 'master' into pca_hotfix
changes variable name and adds test of the edge case
Add test for Landmarks __str__
Merge pull request #365 from jalabort/pca_hotfix

Fixes orthonormalize_against method edge case.
Trilist is optional!
Add tests for trilist case
Merge pull request #366 from patricksnape/tcoords_change

Fix optional trilist in mesh importer