Adds DX, DL, DP, along with documentation.

This commit simply adds the classes. Nothing is changed in the source.
Adds tests for PCAModel.

Tests for:
- changing n_active_components
- trimming
- variance coherence and variance coherence after the previous two
Merge pull request #324 from jabooth/diffstaged2

Adds DX, DL, DP, along with documentation.
Merge pull request #330 from jalabort/pca_variance_trim_tests

Adds some tests for PCAModel
Merge remote-tracking branch 'upstream/master' into rmmultivar

Conflicts:
	menpo/model/pca.py