Merge pull request #67 from nikicat/master

discover command support, plugin support
fixed execute/exec_with_tty exception handling
SourceImage: make contents (and hash) independent from generation host
Bump version: 12.1.0 → 12.1.1
actions.edit: simplified subcommand wrappers
Image and SourceImage are now based on BaseImage, "tag" became a property, "id" attribute removed in favor of getid()
Bump version: 12.1.1 → 12.1.2
actions: added docstrings
actions: convert edit_subcommand decorator to decorator-function
actions: implemented "test" commands for doors and urls