Restore COPYING file with Apache License 2.0

This partially reverts commit 61fa579f9733d64713bcc320d234cefbd68937f7,
except the GPLv3 COPYING file is replaced by the Apache License 2.0.

@ketancmaheshwari <ketancmaheshwari@gmail.com> reported a failure to
build Nimbus in a Ubuntu 12.04 environment because of the removal of the
COPYING file, which is apparently expected by the configure program.

Closes #124.