Refactoring of image I/O to accept data from memory and output to memory

[stian-johnsen]

This is a subtask of ticket #124:
NiftyNet currently can only read images from the F/S and write results to the F/S. In order to make NiftyNet truly useful as a module in 3rd party code, it should be modified such that it can also accept images from memory and write to memory. To this end, this ticket proposes to generalise niftynet.io.image_reader.ImageReader into the general notion of an image source. Similarly, at the other end of the pipeline, there should be an image sink.