Add support for creating new key pairs and certs.

Problem/Motivation

This module requires the user to create the key pairs outside of the module and upload them as PEM file or reference them in files or environment variables. This can be difficult for some users who just need to create a cert for development purposes.

User interface changes

• Add a "New Key Pair" and "New Certificate" link next to "New Key" on admin/config/system/keys.
• Allow the user to select the strength of the key pair.
• Allow the user to fill out the common fields of a certificate
• Allow the user to self-sign the certificate or use another cert+private key to sign.

API changes

Add methods to the KeyPair class to do the heavy PKI lifting.