New runtime interfaces

[arjanz]

Add a more intuitive approach to interface with the runtime functionality, examples:

• Storage functions:

substrate.runtime.at(block_hash).pallet("System").storage("Account").get("13GnsRKEXCAYLJNScBEDj7rHTXkkHAVTj5QMNp6rnyGuTAVN")

• Runtime calls

call = self.kusama_substrate.runtime.pallet("Balances").call("transfer").create(
                dest='EaG2CRhJWPb7qmdcJvy3LiWdh26Jreu9Dx6R1rXxPmYXoDk',
                value=3 * 10 ** 3
            )

• Runtime API calls:

self.substrate.runtime.api("Core").call("version").execute()

• Update documentation
• Backwards compatilibity
• Update unit tests

These interfaces will be available next to the old methods.