4th release candidate for version 0.1

Sadly, a lot of smaller bugs still had / have to be squashed:

• major rework of the import of filter modules and classes, giving more flexibility (and less errors) in the organization of filter modules. The import and filter tree building process now also fails more graciously with buggy filter modules.
• new default filter class 'Manual' for the manual entry of P/Z and coefficients. FIR / IIR can now be selected in the P/Z and the coefficient entry widgets
• make docutils an optional module
• fixed wrong indication of spec violations in filter_info.py
• lots of code and comment clean-up