Add 'make release'
Add 'make release'

Add 'make release' that automatically creates tarballs for all CLI
platforms in _output/release. It also creates a CHECKSUM file using
sha256 sums. Finally, it displays the contents of the CHECKSUM file as
well as the sha256sum of the CHECKSUM file.

Signed-off-by: Andy Goldstein <f9f15225913dfefeeed67d49bbada67d1dd4ac7a@gmail.com>

heptio/ark,heptio/ark