Reshape single input batches for inputs of varying dimension

To feed inputs of varying dimension, the DATA layer reshapes its prefetch and top blobs when the batch size is 1. This is useful for models of variable input size, such as fully-convolutional models.

By the grace of #594 this is a simple change.
I limited it to the case I needed for simplicity, but it is true that one could batch inputs of the same dimension and reshape across batches. If someone decides to do that they can generalize.