cache: introduce 'cache info' command

usage:
 $ collie vdi cache info # Get the cache informatin of the node

Example:
yliu@ubuntu-precise:~/sheepdog$ collie/collie vdi cache info
Name	Tag	Total	Dirty	Clean
test		88 MB	68 MB	20 MB
data		88 MB	0.0 MB	88 MB

Cache size 200 MB, used 176 MB

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
add .S file to cscope index

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>