Provide read accessor to all returned values from JSONRPC.Introspect

Currently `JSONRPC.Introspect` returns `id`, `description`, `methods` and `types`.  Probably `id` and `description` aren't useful, but types certainly is in trying to determine what fields a method accepts, or what is returned.  Rather than add special handling for types like that which exists for `@commands`, I've made any keys returned via `JSONRPC.Introspect` available as instance vars with an associated getter.  This will provide access to any additional introspection keys that are added in future versions.