Update Twitter auth API endpoint to use OAuth Echo

[potomak]

A OAith 1 access token/secret pair can be shared only across clients using the same id/secret, so the current implementation of API's Twitter auth is basically unusable. To let consumers use Twitter auth flow let's implement a new version of the endpoint that supports OAuth Echo.

See also:

• https://dev.twitter.com/oauth/echo
• Tomatoes public API #15 (comment)