Add support for CRLs

[El-Virus]

This PR adds support for:

• Validating an ASN.1-formatted CRL.
• Parsing CRLs.
• Accessing a CRL's data. (+Helper functions for checking if a certificate has been revoked)
• Accessing X.509v3 extensions required for the generation of a RFC 5280 conforming CRL.
• Verifying a CRL's signature against a certificate.

It however does not include support for:

• Building CRLs.
• Encoding CRLs in ASN.1 format.

Nonetheless it provides a base to implement such features.

Includes unit tests.

Helps towards #113.