Remove empty Signer, empty Key, empty Signature, empty strings

[Slamdunk]

Addresses #937 for v5.

This PR removes deprecated:

1. \Lcobucci\JWT\Signer\None
2. \Lcobucci\JWT\Signer\Key\InMemory::empty
3. \Lcobucci\JWT\Token\Signature::fromEmptyData
4. and as a result, \Lcobucci\JWT\Configuration::forUnsecuredSigner

This PR is blocked by #936 and will be marked as ready if and when #936 is merged.

Until then, you can better browse the diff comparing this to #936: Slamdunk/jwt@non_empty_string...no_empty

[BC] REMOVED: Method Lcobucci\JWT\Configuration::forUnsecuredSigner() was removed
[BC] REMOVED: Class Lcobucci\JWT\Signer\None has been deleted
[BC] REMOVED: Method Lcobucci\JWT\Signer\Key\InMemory::empty() was removed

[Slamdunk]

@lcobucci I'll rebase this PR and make it ready for review next week

[Slamdunk]

Sorry for not the cleanest way of doing it, but after the mega refactor of #967 it was tricky to get the two PR separated.

I've rebased this and merged #936 here, as well as expanding non-empty-string basically everywhere.

[Ocramius]

Most excellent! Thanks @Slamdunk!

[lcobucci]

Thanks @Slamdunk!