Adding support for external authentication / access token generation

[kevinterrobang]

I am using passport (node authentication middleware) for my user authentication with an additional Instagram authentication module. As this means I'm managing the authentication and access token externally, I wanted a way to initialize Instajam with an existing token and just use the functional endpoints.

[mikefowler]

Seems legit, @kevinterrobang. If you could address the one comment above I'll merge this.

[kevinterrobang]

How did that get there?! Removed the stray newline from the source and from the non-minified distribution file.

[mikefowler]

Thanks!