Add ability to call Analytics.page.

This allows the user to use the named page tracking API introduced in release 
1.0.0 of segmentio/analtyics.js.

This new API is mentioned in the migration notes:
   (https://github.com/segmentio/analytics.js/wiki/Migrating-to-1.0.0)

Details about the API change are in issue #153:
   (https://github.com/segmentio/analytics.js/issues/153)