Add autosign task, drop support for Puppet 3.x

This release primarily covers changes in #21.

• Adds the autosign::generate_token task
• Fixes all of the spec tests, adds more and moves to using the PDK
• Moves all internal documentation to puppet strings
• Ensures that all parameters are documented
• Removes support for Puppet 3.x
• Adds management of logfile and journalfile permissions
• Adds support for modern versions of Puppet Enterprise
• Updates documentation