add checkpoint/restore to the API

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Guard c/r with API version test

These are only available with lxc >= 1.1, so we should only include them if
we're building against that api version.
Use options struct vs. lots of options for c/r methods
Merge pull request #27 from tych0/checkpoint-api

add checkpoint/restore to the API
minor cosmetic change
Add comments to CheckpointOptions/RestoreOptions