Add sigar interface

* Sigar is now called ConcreteSigar
* Add FakeSigar
* Interface defines CollectCpuStats method

Signed-off-by: Kris Hicks <krishicks@gmail.com>
Add GetLoadAverage to Sigar interface

Signed-off-by: Kris Hicks <krishicks@gmail.com>
Add GetMem to Sigar interface

Signed-off-by: Kris Hicks <krishicks@gmail.com>
Add GetSwap to Sigar interface

Signed-off-by: Kris Hicks <krishicks@gmail.com>
Add GetFileSystemUsage to Sigar interface

Signed-off-by: Kris Hicks <krishicks@gmail.com>