docker/pool: add keys directory

- Added key directory with adding `.gitkeep` file for maintainability
- Remove unneccesary cloning repository process from wercker.yml
Dockerfile: remove test command
Merge pull request #81 from mookjp/add-keys-dir

Add keys directory
delegate git clone function of hook.rb to builder server
res.send_response returns if only the process is succeeded
fix error when access resolve_git_commit
build image in separated workspace each commit-ref, and lock building while other process running
add line removed unintendedly
add system test