Expr: add leaves method to Expr
expr: implement leaves on merge and union
expr: leaves work on scalars too
compute: V compute down/up pattern
server: ExprClient
Merge branch 'server-exprs' of github.com:cpcloud/blaze into server-exprs

Conflicts:
	blaze/expr/tests/test_table.py
compute: V fails gracefully on missing leaves
server: ExprClient raises error on server issue
server: to_tree on Table dispatches to TableSymbol
Merge
Rename Dispatcher.resolve to Dispatcher.dispatch

Companion to https://github.com/mrocklin/multipledispatch/pull/35
Merge branch 'server-exprs' of github.com:cpcloud/blaze into server-exprs

Conflicts:
	blaze/serve/server.py
Merge branch 'md-dispatch-resolve' into server-exprs
Merge branch 'server-exprs' of git://github.com/mrocklin/blaze into server-exprs