Simpler Allow method version, also return * for allow-origin when supplied as * in options

[ryanthecubfan]

Reasoning:

Creation of the options object to pass to the existing Allow() made client code needlessly verbose - a simple version that supplies the three most often used CORS headers makes martini setup code much more readable and clean.

Regarding the * - it seemed unintuitive that whatever origin request header was sent was coming even though my client code was passing a * into the handler. Line 73 change remedies that condition.