Custom rdds: s3 reader

Moving towards a unified i/o architecture. This PR adds a "readers" module, which currently provides an abstraction layer over access to the local filesystem and to s3 via boto. 

Other changes:
* disable a failing test in test_series - not immediately clear what the cause is, but should get fixed
* expose a couple options from the spark ec2 script in the thunder ec2 script
* start moving towards more standardized handling of data paths in the series module
* add subsample and exportAsPngs methods in Images
Yeah, I have a boto-based single file *writer* that I should be ready to push up this afternoon; shouldn't be too difficult to do the same sort of thing for a file reader. 

I'd prefer to handle this through boto if possible rather than bouncing through spark's own file IO, just because fundamentally this particular operation really should be handled single-threaded on the driver, and I don't really want the overhead of another parallelize-collect cycle just to grab a single small file.