Timefilter Auto Refresh - Closes "Support for auto-refresh #1845"

Closes - Support for auto-refresh #1845

Feature:
Enable auto-refresh of the various built in tabs, e.g. Discover, Visualize and Dashboard. The implementation should allow future plugins to respond to refresh events.

Usage:
Upon clicking the top right corner, you will be presented with two tabs, "Time Filter" and "Refresh Interval" (see attached image for details). The "Time Filter" tab allows users to add time filters in the same capacity as they are today. The new "Refresh Interval" tab allows users to select a predefined refresh interval. Upon selecting a refresh interval, all tabs will automatically refresh their content.

Implementation:
The feature was implemented by piggy-backing on top of the existing timefilter object. The timefilter object now contains a refreshInterval. The courier now receives reference to the timefilter object and listens for changes on this refreshInterval. It updates the search interval accordingly. 

![refreshfeature](https://cloud.githubusercontent.com/assets/1840773/5365526/bcfefcea-7fa1-11e4-99ae-8ca43dbc30ed.png)


Personally I'd prefer if this was a falsy check, or a check that `refreshInterval` is infact a valid interval, rather than checking if it is one of the many things it might be.