Various legal related commits

This PR does the following:
 * Establishes the basis allowing dual licenses
 * Ensures the copyright to every source and header file
 * creates a LEGAL.md, creating the idea of voting for issuing licenses
 * Protects the freedoms of users of Tox by ensuring that they use a OSI approved license client
 * Further protect users freedoms by ensuring that Project Tox can not allow any closed source clients
 * Allow Tox to be distributed on platforms that are not GPLv3 compliant while protecting users
 * Protect developers by forcing Project Tox to take action on requests for dual licenses
 * Ensures that all current and future toxcore code, no matter how minor, is GPLv3
 * Allows Project Tox to further improve Tox in ways not yet known by assigning copyright correctly

Note: Do not merge till I've gotten contributors who've added code to a file without a header previously to agree to copyright code to Project Tox, as established by LEGAL.md.