Added documentation for selectable aggregates

Documented available aggregate functions
and added example syntax showing how to use
improved Statistics API, which allows the user to
selectively access aggregate functions. Also
documented the Capabilities API, which allows
the user to find out what functionality is supported
by their storage driver.

Closes-Bug: #1290924
Change-Id: I9739d2102baf6040de017843ae20391c4c9b6fff
Merge "Added documentation for selectable aggregates"