host memory stats: update API.md

Add 'memory' field to host stats.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
host memory stats: update model and mockmodel

get buffers, cached, free and avail memory by psutil.
avail = free + buffers + cached
http://code.google.com/p/psutil/source/browse/psutil/_pslinux.py :175

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
host memory stats: update test case

update test_model and test_rest

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>