A bunch of updates:

* Better performance for pagerank/dot product:
  Use tocoo() before sending to reduce network/pickle overhead
  Use parakeet to initialize random matrix

* Support for sparse updates to dense tiles

* Change interface to running kernels:
  Defer waiting for updates from kernel functions until all
  kernel functions finish on a worker.

* Worker helper process shutdowns after all RPC processes go away.

* Add add/sub to builtins.  sparse_diagonal.

* Add tests for updating tiles in various forms.