Updates ohShopify User Agent

HAC isn't a descriptive UserAgent string. Updating the UA to be something that explains what it is.
Merge pull request #1 from csaunders/patch-1

Updates ohShopify User Agent
add validateSignature function
add validateSignature example to README
Merge pull request #2 from apeschar/signature

validateSignature function
Merge remote-tracking branch 'upstream/master'
set tls 1.2 transfer mode per recent API change