Adds str and trim by variance to pca.

- Reimplements noise_variance property, and adds some convenient properties related to variance
Modifies trim_components based on Nontas commmets.

- No error is raised if n_components is bigger than the current number of components or bigger than the current amount of variance. This avoid stupid parameter setting errors when building deformable models such as AAMs or CLMs.
Raises error if n_components float and > 1.0
Merge remote-tracking branch 'upstream/master' into pca_str_trim

Conflicts:
	menpo/model/pca.py
Merge pull request #315 from jalabort/pca_str_trim

Adds str and trim by variance to pca.