add multipage tif loader and test
add test for loading multiple tif files
expose 'ext' in user-level image loader method calls, minor cleanup
move spark context parameter to image loader init from method calls
move dims param to ImageLoader fromStack from init, remove unused valuetype
support optional 'ext' keyword arg in ImageLoader
change ImageLoader fromMultipageTif to return k,v pairs of fileindex, concatenated numpy array of multiple pages
Merge remote-tracking branch 'upstream/custom-rdds' into custom-rdds
Merge pull request #27 from industrial-sloth/custom-rdds

Custom rdds: support loading from multipage tifs