Go-self-update lacks support for checking integrity of downloaded files.
For more advanced situation it's necessary to validate the hash or
verify against public signatures. This patch adds support for
SHA2 hash and ECDSA PublicKey signature validation.
SHA2 uses file with suffix `.sha256`, whereas ECDSA uses
`.sig` file endings. See `selfupdate/validate_test.go` for examples.
Signed-off-by: Tobias Kohlbau <t.kohlbau@myopenfactory.com>
