shape dispatches to datashapes
add dataset draft
compute_up on dicts
server works on single data source

That data source may be a dictionary
Data becomes function, replaced by InteractiveSymbol
resource_pytables accepts one argument
Data translates ::/datapath to fields
pre_compute on multi-datasets applies selectively
update interactive docs to focus on engine
simplify Dataset
rename Dataset to CachedDataset
move dataset.py to cached.py
import CachedDataset in __init__
Minor fixes to server and cached

response to comments by @cpcloud
server: support :leaf special string
update server docs for non-namespaced server
docs: server advanced use
docs: caching
add doctest-skip tags to server docs
skip hash-dependenct doctests