Nested tokens

[Spomky]

Q	A
Branch?	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	#65
License	MIT
Tests added	yes
Doc PR	web-token/jwt-doc#4

This PR fixes #65 by adding a NestedTokenLoader.
The loader is able to decrypt and verify a nested token i.e. a signed and encrypted token.

Example of Symfony configuration:

jose:
    nested_token:
        loaders:
            nested_token_loader:
                signature_algorithms: ['PS256']
                key_encryption_algorithms: ['RSA-OAEP']
                content_encryption_algorithms: ['A128GCM']
                jws_serializers: ['jws_compact']
                jwe_serializers: ['jwe_compact']
        builders:
            nested_token_builder:
                signature_algorithms: ['RS256']
                key_encryption_algorithms: ['A128KW']
                content_encryption_algorithms: ['A128CBC-HS256']
                jws_serializers: ['jws_compact']
                jwe_serializers: ['jwe_compact']

Usage:

$jws = $loader->load($token, $encryption_key_set, $signature_key_set);

Remaining tasks:

• Create a NestedTokenBuilder
• Write documentation

[coveralls]

Coverage increased (+0.9%) to 85.717% when pulling ce810b1 on NestedTokens into 7055ece on v1.1.