Merge pull request #2 from rgee0/bumpv2

Append v2 to module path

Add Go module

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Update Validate to expect "sha1=" prefix for hash method. Other

variants should be supported in the future.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Add "Sign" function

Sign function allows HMAC package to be used to generate hashes as
well as for verifying them.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Add tests for empty hub-signature

Hub signature is required but when it was empty it was passing
validation. This adds tests and fixes the problem.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>