reorg: scripting + JSON

The scripting system has been moved from quorum/script to the delta package. The number of global variables has been reduced, but only by consolidating the variables into a global environment struct.

The RPC server and client now use JSON. Gob had to be abandoned because it is slightly non-deterministic, which means the hash of gob-encoded data can change. Down the road, a more efficient binary format will be used. JSON was chosen because its RPC bindings were already provided in the standard library.

The next step is to remove all GobEncode/Decode methods. Where necessary, they should be replaced with MarshalJSON and UnmarshalJSON (**not** Marshal and Unmarshal), so as to implement the json.Marshaler and json.Unmarshaler interfaces.