GREATLY simplified cache class (now FixedFifo)

- the previous cache class (CacheList) was suffering from a lot of
bloat, and was trying to be too many things.

- it has been replaced with a fixed-length fifo, which does only a
couple things, but does them concisely and well.

- all reading/writing is no longer handled specifically by the
cache class, as it is not part of the functionality of the cache
class itself

- pickling is now used to write/load the recent items fifo to/from
disk.

this closes issue #56