DOC: clean up top-level files: THANKS, COPYING, CHANGES

[rgommers]

• Remove some outdated files that we no longer need.
• Move COPYING to LICENSE, is more commonly used and more clear imho.

[grlee77]

+1
I don't think any of the info in CHANGES.txt prior to 0.3.0 is in the release notes, though. Do you think it is worth keeping it around in pre-0.3.0.rst or something like that in the doc/release folder?

[rgommers]

Not sure that's still interesting - any release pre 0.2.2 is so old that no one will care. So keeping this in the commit history is probably good enough. Not a strong opinion though, I can add it if you prefer.

[grlee77]

sounds reasonable. merging as-is. thanks