Enh: New ShinkenPlugin base class.

+ Moved STATES to own module for easy reuse.
+ custom PluginParser for customized exit().
+ better add_threshold() functions.
+ helpers with parsing methods for "size" amounts (MB|KB|..).

+ included argparse if one doesn't want to install it from system packages.
Adapted TestPlugin to execute new ShinkenPlugin.

+ make sure SystemExit is raised.
+ check stderr output.