Introspectable Extrinsics, Events & Storage plus an RPC

[gavofyork]

All information in decl_module! and decl_storage! should be replicated into a call that returns it in an easily accessible structured format (JSON/YAML/whatever).

• Metadata creation for extrinsics;
• Metadata creation for storage;
• Metadata creation for events;
• RPC call to return the metadata.

[gavofyork]

Note that events is incomplete:

r#"{ "name": "TestEvent", "items": { "#,
	r#""system": "system::Event", "#,
	r#""event_module": "event_module::Event<TestRuntime>", "#,
	r#""event_module2": "event_module2::Event<TestRuntime>" "#,
r#"} }"#

This is wrong. Like Call and Storage it is not enough to merely list the modules: the JSONRPC client has no idea what to do with system::Event or event_module::Event<TestRuntime>". It must detail the contents of these enums. Just like for Call, for this you'll need to encase the pub enum Origin declaration in a macro which generates the inner json.

[bkchr]

As #710 is closed, we can mark this issue as closed/resolved?