Excel addin and scala rpc server

Seems to work fine but pretty unpolished. The addin connects to the
RPC server to get a list of methods which it exports to excel. Calls
to the exported functions are serialised into json and sent over
http to the RPC server.

All functions are exported to excel as threadsafe so may be called
in parallel.

updated readme
Merge pull request #1 from og3b/master

Excel addin and scala rpc server