BaseChecker: Modify to correspond with all base classes

Use classmethod decorator for BaseChecker methods.
Add match method.
Add dictionaty with check_tools
Add decorator to add new check_tools

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Modify the Pkgdiff checker implement checker base class interface

Removed PkgCompare class and replaced it with generic Checker class.
Added get_supported_tools() into Checker.
Catch NotImplementedError in the application when creating the checker
instead of checking tools in advance.

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Application: return non-zero value on error

Signed-off-by: Tomas Hozza <thozza@redhat.com>