Major revision of the interfaces of layout classes.

- Removed unused return's
- Fixed PEP8
- Underscored private attributes
- Removed unused attributes
- Added @property to expose public attributes
- Added numerous comments to improve readability
Fixed failing test.

It was failing because of encapsulation.
Added test case for LTComponent.
Made LTTextContainer accept only LTText instances.
Added first test to LTChar.
Made unittests in travis be discovered.
New feature: LAParams is now responsible for group creation.

One can now subclass LAParams to implement custom algorithms to
merge characters into lines, lines into boxes, etc.

The existing public interface didn't changed, only new interfaces
were exposed.