Base64 backward compatibility and performance

[sleptor]

Related to #397

After replacing the spooky-labs/base64url library with another one, you lost backward compatibility since the new library can't decode standard base64 (non-URL safe). Also, the new library is much slower since it uses a pure PHP implementation of the base64 decoding algorithm.

This PR contains tests only, which are now falling on the master branch.

I would to borrow code here (from the old library) https://github.com/Spomky-Labs/base64url/blob/v2.x/src/Base64Url.php
and place it directly in the package, and refactor the code, of course.

The ideal solution would be to extract Base64 encoder as a dependency while adding the possibility of passing a custom encoder as a parameter.

Pseudo-code:

$myEncoder = new StandardBase64OnlyEncoder()
$wp = new WebPush();
$wp->setBase64Encoder($myEncoder);

However, it will require deep refractory; for example, the Encryption class must not be static anymore.

[Minishlink]

Thanks! I reverted the problematic PR and will merge this in case there is enough arguments to switch over to paragonie (or adding this base64 encode/decode interface)