Implemention of more methods to make biscuit-php usable

[alyrow]

I have implemented some more methods to make this php binding usable:

• Added methods from_base64 and to_base64 into Biscuit class for allowing some de/serializations
• Added method authorizer into Biscuit class to get an Authorizer object in order to test our rules on the biscuit
• Added method build into BiscuitBuilder to get a Biscuit from a BiscuitBuilder
• Changed constructors, from_... and to_hex methods in KeyPair, PublicKey and PrivateKey classes to handle strings instead of bytes since in my tests I was unable to load a private key from a string
• Added method query into Authorizer class. It's more an hack than a functional thing but I don't have the time to do it properly: instead of returning a list, it returns a string like this: "term1"|"term2"|3

[ptondereau]

Hey @alyrow thanks for your awesome PR!
Before merging this PR, it would be more consistent to upgrade biscuit deps to v3 instead of alpha one and also ext-php-rs to v0.10

[alyrow]

Yep sure

[ptondereau]

Thank you, looks good, just need to fix php tests