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.

Let's specify these locations by referencing off paths to modules within Thunder, rather than by searching the python path.