Add Evernote (and Evernote Sandbox) provider

Added an “Evernote” and an “Evernote (Sandbox)” provider.

I added a small statement to the `LICENSE.md` file which is required by Evernote (see [Evernote’s trademark guidelines: “Attributing Ownership of the Evernote Trademarks”]((http://evernote.com/trademark/)).

There is also one change to the lib itself: Evernote doesn’t use the token credential secret and will therefore return an empty token (empty string). `oauth1.coffee` now allows an empty `oauth_token_secret` as long as it’s present.

Edit: To validate these two providers correctly, the changes from #41 are needed.