Add the _types module.
Add the verschemes.future package to setup.
Import the usual stuff from __future__.
Add documentation.
Standardize the use of the 'future' package to globally support Python 2 & 3 with the same code.
Change SegmentField.re_pattern.
Add SegmentDefinition.separator_re_pattern.
Fix Pep440Version separator and normalization.
Improve documentation.