add "lxc-snap -l base"

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxc-snap: snapshot: write timestamp to 'ts'

Instead of one big /var/lib/lxcsnaps/data file containing the
base container, snapshot name, and timestamp, put the timestamp
into /var/lib/lxcsnaps/$name_$i/ts.

Likewise a commit message, if provided (as a file) at snapshot
time, will go into /var/lib/lxcsnaps/$name_$i/comment
(not yet implemented)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>