Enhancements on indentation code and related functions.

Enhancements on indentation code and related functions.

This commit includes:

    * A more robust implementation of `python-indent-calculate-indentation'.
    * enhancements on `python-indent-context' when dealing with backslashes
      and blocks.
    * Many changes, comments and enhancements to
      `python-indent-calculate-indentation'. Many of them especially focused
      to match pep8 guidelines, being this one the most important new one:
      http://mail.python.org/pipermail/python-dev/2011-June/111760.html
    * Better `python-info-line-ends-backslash-p' that would work as intended
      on narrowed buffers.
    * `python-info-continuation-line-p' now does what's supposed to do.
    * Enhanced implementation of `python-info-continuation-line-p',
      `python-info-block-continuation-line-p' and
      `python-info-assignment-continuation-line-p'

New Functions:

    * `python-util-forward-comment' a simple replacement `forward-comment'
      with some necessary enhancements.