Add memory data layer to pass data directly into the network

As stated in the big umbrella issue #148, current version of DataLayer is bound to specific data source and data format. Ideally data from any external sources and in any format should be able to stream though the network. This PR provides an in memory gateway for such a purpose.

Although the memory data layer can be straightforwardly integrated with #120 to enable feeding raw images in memory into the network, it simply copies memory blobs at the moment. If anyone is interested in such a design, I will further refine and extend it.