CRM-13925 - Generate license files

This adds three new options:
 * --license (default: AGPL-3.0)
 * --author (default: read from ~/.gitconfig)
 * --email (default: read from ~/.gitconfig)

The values will passed through to info.xml and LICENSE.txt. The content of LICENSE.txt will be determined using a new library, https://github.com/totten/license-data .