Adding filesystem usage calculation for docker containers. This patch also includes some internal refactoring.
'machine' api now exports a list of all existing 'ext*' filesystems along with the capacity.
Avoid storage usagge calculations when aufs driver is not being used.
Merge pull request #267 from vishh/disk_usage

Add filesystem usage support for docker containers.
LXC Factory and Handler

This factory and handler can retrieve memory, cpu, diskio, and network information of lxc containers.
The handler can be passed additional container information(e.g. network interface) using a cdesc.json file
NetworkInterface now contains both VethHost and VethChild
Updated import and comments
Added network stats to raw handler

Raw handler now parses an optional json file with the network interface information, and emits network stats.