Remove 99designs/http-signatures dependency for stability

[hannesvdvreken]

Removed

• Dependency on dated version of 99designs/http-signatures
• HMAC class, inlined the functionality into Signer class.
• public Signer::urlSafeB64encode method. Was not supposed to be used as part of public interface of this package.
• signature_middleware_factory global method in helpers.php file.

Changed

• Batcher constructor (was not supposed to be used outside of $client->batcher())

Added

• public method Signer::signRequest which handles Request signing for Batcher.

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Done this because of GetStream/stream-laravel#65. Now we can install next version (2.6) together with Laravel 5.6 and Symfony 4.0. (Symfony 4, dependency of Laravel 5.6 was not compatible with previous version of 99designs/http-signatures)

This way we don't need to tag new major version (v2.5.2 -> v3.0.0) because of upgrading an underlying dependency.

[dwightgunning]

I can't say too much for functionality / semantics. Should we sit together and install / run / test it on my machine?

Visual coding style check passes for me.

[hannesvdvreken]

@dwightgunning All good 👍 did all the checks locally to ensure it does install nicely with Laravel 5.6 and Symfony 4.0, which was the goal. And all integration tests succeed as well, and did some extra manual tests too.