Custom rdds: support mixed data types in series loader, add image loader tests

main code changes here are in SeriesLoader.fromBinary, supporting data types beyond int16. 
Also the following minor fixes:
* thunder script now searches for py4j jar rather than assuming hardcoded version (version changed with 1.1.0 Spark release)
* Images.dims is now lazily evaluated on first use instead of on object instantiation. This requires an expensive first() call.
* the collect() method has been renamed from "collet()"
* bugfix: ImagesLoader.fromFile now specifies arguments as sc, reader, consistent with its uses, rather than reader, sc.

Test cases and data added for SeriesLoader.fromBinary, ImagesLoader.fromTif, and ImagesLoader.fromPng.

Nice, didn't realize ``dtype`` could take strings.