Stop raising defunct processes

Try to fix #53 .

- [x] Add API to access docker like docker/pool/builder/lib/builder/git_handler.rb
- [x] Add API server for docker to EM
- [x] Change hook.rb to get values via these API then remove "`"

I fixed the points below;

**docker/pool/Dockerfile**

- Replaced `FROM` image to [prevsio/poolbase](https://github.com/prevs-io/docker-pool-base).
- Moved depended package installation process to `docker-pool-base`
- `curl` with `-s` option to suppress output

**docker/pool/builder/bin/build_server**

- Add `DockerHandler` http api server to eventmachine

**docker/pool/builder/lib/builder/config.rb**

- Add `read_ids` method for reading commit-id/image-id pair from `/app/images/ids`. this method is moved from `hook.rb`

**docker/pool/builder/lib/builder/constants.rb**

- Add the constant of `/app/images/ids`

**docker/pool/builder/lib/builder/docker.rb**

- Utility module for accessing preview containers in pool environment
- `find_container_by_commit_id` method takes git commit id, returns address (with port number) of matched container
- the port number of container to preview is fixed; `CONTAINER_PORT=80`

**docker/pool/builder/lib/builder/docker_handler.rb**

- DockerHandler is http server to communicate the container infomation with `hook.rb` in mod_mruby
- http server has the endpoint `/containers/<git-commit-id>`, and for example, `hook.rb` calls `http://0.0.0.0:9002/containers/a12314zbvd` by `GET` request, then the server returns response `{:status => 'success', :addr => 'xxx.xxx.xxx.xxx:80'}`with json format. If the container corresponding to git commit id doesn't exists, the server returns `{:status => 'error'}`

**docker/pool/handlers/hook.rb**

- Shrinked the codes.
- moved procedure resolving container address by git commit id to `DockerHandler` server.
- Stop to executing external command with "`" syntax.
- Instead using `curl` command to communicate with http server, use mruby module [mruby-webapi](https://github.com/iij/mruby-webapi)

**scripts/init_host_server**

- replace cache file
@ainoya This method returns address of container so it would be better to change the name or comment.