Channels flip

This pull request moves the channel axis in `menpo` `Image` (and subclasses) from the very back to the very front, i.e. images are now defined to have shape `(n_channels, X, Y, ...)` instead of `(X, Y, ..., n_channels)`

- This change allows for a much faster gradient computation, specially in the case of 2-dimensional multichannel images.
- The previous should, in turn, lead to faster feature computation (for those features that needing to compute gradients) and much faster AAMs in `menpofit`.

> As an example, in my machine, the gradient computation of an old `menpo` `Image` of shape (206, 213, 36) took ~ 55ms.  The gradient computation of the equivalent `menpo` `Image` with the channels at the front (36, 206, 213) takes now ~17ms.