Runtime: Signature and sender should be Optional

[gavofyork]

Currently they rely on Default trait to be implemented which is annoying. Rather they should be Option<Signature> and Option<AccountId>.

[rphmeier]

addressed in paritytech/polkadot#169 using a wrapper type that imposes the Default bound.

[gavofyork]

Not actually addressed; this needs sorting properly so that signature and sender are either both Option or not. In either case, they should work with an arbitrary lookup scheme and, probably, arbitrary crypto for the signing.

[rphmeier]

Using Option is fine but we can get away without the runtime overhead if we use a trait-based system. There should definitely be no overhead in runtimes where there is no such thing as an optional signature. i.e. the demo runtime or a simple smart-contract runtime