Added new documentation page for frameworks.

 * Consolidate framework list on a new page
 * Add short description to each framework
 * Remove frameworks from home and powered-by, link to new page instead

Review: https://reviews.apache.org/r/18328
Logged ZooKeeper session expiration event in group.
Added hostname attribute to framework and displayed it in master's web
ui.

Review: https://reviews.apache.org/r/18361