Improved base code of pdfminer.

- Fixed all PEP8 errors
- Moved most tests to folder "tests" (discoverable by unittest)
- Added some docstrings
- Removed redundant `return`s
- Removed redundant comments
- Removed reduntant parenteses
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.
Added __getitem__ to LTContainer.