ignored .idea directory
downgraded requests to 2.2.1 to match docker-py dependency
Removed 'parse_repository_tag' function as it now exists in the upstream docker library.
Support for fetching images that appear in a pipeline.

Runner interface now has `install_tool()` method that individual runners can implement.
JobNode and JobGraph now have install_tool and install_tools methods, that delegate to appropriate runner method.
CLI has install subcommand.
pep8 fix
Merge pull request #7 from rabix/feature/predownload-images

Feature/predownload images