Fixes references to depreciated goauth2 libaray and replaces example
with the recomended oauth2 implimentation.

- minor updates to README
- expand package docs to include more information on authentication, as
  well as new sections for rate limiting and setting pointer values.

Now that we're using anonymous struct fields for Rate, go 1.1 really is
required.

This now aligns with the (pretty strong) golang convention of the
package name matching the path basename

also minor doc cleanup