Refactor use of PynagError in Control
Refactor use of PynagError in Command
PynagError: Fix call to super

Call to super was errounously done as self.__class__ instead
of explicitly saying 'PynagError'.

This lead to recursion errors when multiple levels of subclassing
this error.
Refactor PynagError in the Utils package

All modules that raise error now have their
own exception that inherits PynagError.
Refactor PynagError in the Plugins package

All modules that raise error now have their
own exception that inherits PynagError.
Exception handling in Parsers improved

* All exceptions moved to the top of the file
* ParserError now inherits from PynagError instead of Exception
Unify exception handling in Model

Exceptions that previously raised PynagError now all
raise a subclass of PynagError instead.
Minor fixes after running pyflakes
Merge pull request #191 from pynag/refactor-pynag-errors

Unify Exception Handling