add Rotation.from_2d_angle, Image.rotate_about_centre
visualize image on MaskedImage reports Mask proportion
path is now preserved on image warping
Merge pull request #509 from jabooth/pathpreserve

path is now preserved on image warping
Merge pull request #508 from jabooth/maskediamgeprop

visualize image on MaskedImage reports Mask proportion
all menpo io glob operations are now always sorted
Merge pull request #510 from jabooth/sortedorderimports

all menpo io glob operations are now always sorted
Merge remote-tracking branch 'upstream/master' into rotate
tidy signatures, added documentation
Merge pull request #511 from jabooth/rotate

2D Rotation convenience constructor, Image.rotate_ccw_about_centre