Add COPYRIGHT file in top-level directory

Also include lua MIT full license text in COPYRIGHT file,

lua claimed(http://www.lua.org/license.html):

"The above copyright notice and this permission notice shall be included
 in all copies or substantial portions of the Software."

So only copyright notice included in each file header is not
enough.

Note ktap is licensed as GPL v2, this is compatible with MIT license,
and confirmed with official lua team(team@lua.org).

Original discussion:

>> what's this "compatible with GPL" really means?
>> Would I redistribute ktap as GPL licensed with lua MIT Copyright notice put in somewhere? or use "Dual MIT/GPL" license?

> Both solutions are fine.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>