Token authentication

[graft]

Token authentication takes a signed JWT, validates it using a public key and the expiration timestamp, and creates an Etna::User and adds it to the rack request object.

If the authentication fails for any reason, the user gets a 401.

The Etna::User exposes a basic permissions methods API (can_edit?(project_name), etc.) which controllers can use for checking if the user is valid.

[graft]

In d0406ff I add Etna::Auth and Etna::User - the former checks the existence of the token and validates it, then creates an Etna::User object that the controller can use for checking permissions.

[graft]

In b9e12fc I add a Etna::TestAuth rack layer which can be used in place of Etna::Auth for use with testing.

[graft]

Fixed by #2