Add period support in storage drivers and API

This modifies the API to accept a period argument defining on which number
of seconds the statistics must be grouped. This also changes the return type
of the /statistics endpoint to always return an array of statistics.

Change-Id: Iac492f5daf600bd1653da5c95e4184e778978d35
Signed-off-by: Julien Danjou <julien@danjou.info>