Concat layer

Added layer to concatenate blobs along one dimension. For now it only allows to concatenate along `num` `concat_dim=0` or `channels` `concat_dim=1` dimension. This layer can take multiple blobs (at least two) and produce one that is the concatenation along one dimension of them. The other dimensions must agree.

It adds `optional unit32 concat_dim=65 [default=1]` de  to the list of params of caffe.proto. By default it concatenate blobs along channels.