add Reshape support to PoolingLayer
add Reshape support to LRN layer

Note that WITHIN_CHANNEL LRN supports Reshape, but is not very
efficient, just recreating the internal layers. This could be fixed
with some rearranging of code.
add Reshape support to SoftmaxLayer
test net reshaping
don't reallocate blobs when shrinking memory use

This allows nets to be reshaped very quickly (essentially for free) as
long as sufficient memory has been allocated. Calling Blob::Reshape in
order to free up memory becomes impossible; however, this is not a
normal use case (and deleting blobs does free memory).