add server to blaze/__init__.py
Server from_tree checks compute_one to find exprs

Previously The server gets a string like `'By'` and queries this
against `blaze.expr` to get the class `By`.

Now we do this but, if it fails, we then check compute_one signatures.
This has the virtue of using the dispatch system to pick up foreign
expressions.
Merge pull request #631 from mrocklin/server-with-foreign-expressions

Server with foreign expressions