Implement some auth

[meatballhat]

From chat:

[6/20/14, 11:17:01 PM] Josh Kalderimis: sure, i am thinking pub/priv key, travis-build uses pub key to sign the job id and timestamp, artifacts server uses the priv key to decrypt and check the job id and timestamp, +- a minute
[6/20/14, 11:17:14 PM] Josh Kalderimis: this is only for posting info
[6/20/14, 11:18:00 PM] Josh Kalderimis: the other auth would be based on the travis token, and the artifacts server checking permissions to get and list based on travis-api confirming the token has access to the repo