Style Checker for Sphinx docs

Hi all

This <https://github.com/harlowja/doc8> is a nice tool to check following
styles for docs:

- lines should not be longer than 79 characters - D001
- exception: line with no whitespace except maybe in the beginning
- exception: line that starts with '..' -- longer directives are allowed,
including footnotes
- no trailing whitespace - D002
- no tabulation for indentation - D003
- no carriage returns (use unix newlines) - D004

Although we already have tool to strip whitespaces, we can use it for other
three things.

Regards
Sudhanshu Mishra