oauth-encryption code review items

Have `OAuthEncryption.open` throw "decryption unsuccessful" on any
error to avoid allowing an attacker to break the encryption key by
observing the result of sending manipulated ciphertexts.

Add oauth-encryption and the oauthSecretKey config options to the docs
page.

Update history.

More documentation on functions.

Use `OAuth.sealSecret(...)` instead of `{seal: ...}`.

Rename OAuth._openSecret(s) to OAuth.openSecret(s).

In the readme describing how to generate a key, just use Node instead
of going through the hassle of creating a Meteor application.

Have the `oauth` package weakly depend on `oauth-encryption` for when
it's being used without accounts.

Add tips on using oauth-encryption without accounts to the readme.