Added Permute layer as suggested by loyeamen on #401

[anayebi]

I added the Permute layer as a Core layer, as suggested by loyeamen on #401. It permutes the dimensions of the data according to the given tuple.

[fchollet]

Can you add documentation, including a very short example (as part of the documentation) of the use case of this layer?

[anayebi]

Sure, I'd be happy to. Would you like me to add it to the keras.io documentation page?

Anyway, here it is below:

keras.layers.core.Reshape(tuple)

Permutes the dimensions of the data according to the given tuple.

Input shape: This layer does not assume a specific input shape.
Output shape: Same as the input shape, but with the dimensions re-ordered according to the ordering specified by the tuple.
Arguments:
Tuple is a tensor that specifies the ordering of the dimensions of the data.

Example: # input shape: (nb_samples, 10)
model.add(Dense(10, 50)) # output shape: (nb_samples, 50)
model.add(Reshape(10, 5)) # output shape: (nb_samples, 10, 5)
model.add(Permute((0, 2, 1))) #output shape: (nb_samples, 5, 10)

[fchollet]

I modified it to exclude the samples dimension form the permutation, as it would break training. I added the documentation.