Remove support for Python 2.5 to clean the code

  - update (part of) the documentation
    - some URLs for Windows need to be updated
  - update classifiers in setup.py
  - remove `from __future__ import with_statement`
  - use `{k: v for ...}` instead of `dict((k, v) for ...)`
  - remove hacks for `gcd()` and `binrepr()` implementations
  - use class decorators
  - remove Python 2.6 Travis-CI tests