Major refactor of os and installer plugins and added install verb

[NikolausDemmel]

Following recent discussion, this PR refactors install and os plugins. Based on that the resolve is given a major overhaul and install is implemented.

Some more concrete changes:

• removed "priority" based resolution and replaced with notion of "core" and "addiontal" installers
• installer and os plugins are now based on plugin_utils and have a more uniform API (preferring properties over getters). New base classes InstallerBase and OSBase implement some common functionality (while the abstract base classes Installer and OS contain no implementation).
• both plugin types have options, which are passed through from the config files and utilize config_utils for describing the structure and parsing
• only the normal info output goes to stdout, everything else like error, warning and info_v goes to stderr
• installer plugin now have a "prerequisite" abstraction for checking things like if the installer is present, if it is updated, and if resolution-specfic options are met (e.g. taps for homebrew or repositories for apt)
• installer plugins use config_utils to parse installer_rules reusing that code for improved error handling and warning the user about unknown keys (in the installer_rule)
• added option to disable specific plugins
• added xubuntu OS plugin to showcase how derivative OS's with matching versions are supposed to work
• reorganize tests to run in order unit tests, system tests, code style tests
• improved the handling and printing of "chained exceptions"
• all exception classes are now defined in their corresponding modules
• introduced XylemInternalError to wrap errors that are likely bugs, not user/config/environment errors
• improved top level error handling:
  • at the top level XylemErros are caught and printed showing chained exceptions
  • no tracebacks are shown, except in debug mode, or for XylemInternalErrors

[NikolausDemmel]

@wjwwood: didn't get it done in time yesterday, but here it is.

[wjwwood]

Looks good to me.