NF: class SpooledEphemeralFile

Hi Louis,

Very nifty package you have here! I'm not sure if you are taking pull requests. I was inspired by your approach and so put together a class that inherits from EphemeralFile, trying to keep everything in RAM. Its a branch in my fork, and uses a SpooledTemporaryFile.

- try to keep everything in RAM (so if RAM swaps out to disk, its encrypted)
- smooth out readlines() recursion issue
- require a 256-bit key
- probably inefficient implementation, readlines is slow
- a few minimal tests

Best,

--Jeremy