add --strategy to choose the strategy to use

Signed-off-by: Victor Vieux <vieux@docker.com>
simplify

Signed-off-by: Victor Vieux <vieux@docker.com>
nitpick

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge pull request #168 from vieux/strategy_flag

add --strategy to choose the strategy to use