Cleanup Makefile.

- The "--no-site-packages" option is default now
- "pip install -e" is basically equivalent to "python setup.py develop"
- Delete also dist/ and MANIFEST
- Delete all *.py[co] files (and __pycache__ directories on Python 3)
Update installation docs to mention about pip.

Other changes:

- Minor markup and style changes
- Use https in URLs
- Remove the "python setup.py develop" part from the
  documentation. It's useful for development.
Merge pull request #752 from berkerpeksag/cleanup-makefile

Cleanup Makefile.
Remove an outdated paragraph about getting CPU information.

Python 2.5 or older versions are not supported by Gunicorn.

Also, fixed a typo: mentionned -> mentioned
Merge pull request #753 from berkerpeksag/improve-installation-docs

Update installation docs to mention about pip.
Merge pull request #754 from berkerpeksag/sysconf-outdated-info

Remove an outdated paragraph about getting CPU information.