Code refactor and PEP8 compliance

Create a new class TwrObjectHelper that contains
all repeated code.

Higher-level classes now are sub-classes of
TwrObject.

Removed unused exception types.

Fix all PEP8 compliance errors.
Fix LGPL header and PEP8

Add LGPL header in all source files.

Fix more PEP8 compliance errors.
Update TODO
Release 0.3.0