Custom rdds: image to series conversion

Major functionality added is converting Images to Series by swapping the dimension represented by individual files; so for instance, going from an Image where a single file represents a volumet image at a single point in time, to a Series where a single file represents a z-slice (for instance) across all time points.

Other changes:
* Image tests pulled out into test_imagesrdd.py
* Parameters using in loading particular data sets moved from Loader __init__ methods into individual load() methods
* add thunder.rdds to setup.py and clean up file whitespace
* for loading binary image stacks, change parameter priority order from "config file settings take priority" to "arguments to method call take priority"