Added first steps towards translations

I've added the textdomain loading, but I need to work a bit more on the
functionality and refactoring before I can test this. Also all
translation function calls need to use the $this->localization_domain
instead of the hardcoded 'license' string.
Added some comments for refactor

Temporarily added some comments to make my life easier ;)
Added license per network, site, user or content

This is a huge commit (work in progress)  which adds the following functionality:

- In a WordPress Multisite (Network) installation setting a default
  license for the whole network. All sites of the Network will use this
  license as a default unless the superadmin of the Network install
  allows siteadmins of the sites belonging to the Network to change
  their license.

- Setting a license for a single WordPress install or if the superadmin of the Network
  allows it a site in the Network.

- If a siteadmin allows it, users may choose their own license

- If a siteadmin allows it post or pages may be licensed individually

This is the first step towards having a license per network, site, user
or post/page and needs more work especially in the displaying the
correct license as well refactoring old code and testing it does not
break backwards-compatibility (for now it probably does).