Add: Give a better report on unexpected errors and allow to give explicitely an args array to the plugin.

Propose to submit bug report.
Cleaned shinkenplugins.BasePlugin

- Better exit()
- Use unknown() method where possible
- Use self.x instead of self.__class__.x
- Better use of usage()
Enh shinkenplugins package with a unique version string.

Use a VERSION text file, read by setup.py and by package itself.
shinkenplugins: VERSION file need to be added as package_data.

Also import __version__ from package's __init__.
Plugins requirements: requires shinkenplugins>=0.1.4 (latest) but < 0.2 !

To prevent futur conflict when shinkenplugins lib will have been rewritten.