Thumbor does centerCrop

By default, such issue was closed.
Added test for centerCrop behaviour.
Remove centerCropRequestsAreNotTransformed as centerCrop requests must be transformed.
Merge pull request #428 from eveliotc/thumbor-does-centerCrop

Thumbor does centerCrop
Document empty path throws. File equivalent call.
Merge pull request #429 from square/jw/doc-tweak

Document empty path throws. File equivalent call.