Switched to support arbitrary slices.

Slices support deferred (accumulated) operations.
Switch to python/cpickle messages for now.

Not really that much slower, but definitely easier to deal with.
Switch to dispatching kernels using slices; this allows us more
flexibility for launching jobs.

Moving towards having ArraySlice take over most operations from
ArrayTable.
Merge branch 'master' of github.com:rjpower/sparrow