setuptools' instal_requires instead

As described by http://docs.ansible.com/intro_configuration.html#library the library supports multiple paths to load modules from. 

Currently `setup.py` treats something like `/usr/share/ansible:/my/custom/modules` as a single directory name and results in the installed modules not being on the module path. (instead they are in a directory named `/usr/share/ansible:/my/custom/modules`) 

This commit simply takes the first listed directory as the install directory for the core modules.

This reverts commit 15b89b45, reversing
changes made to 3d836a1a.

use DEFAULT_MODULE_PATH in setup.py so that there is a chance to override with ANSIBLE_LIBRARY env variable

Author: Ton Kersten <github@tonkersten.com>
Date:   Mon Apr 29 10:44:30 2013 +0200

    Fixed the installation of library directories

    When running install the `setup.py` give error

    changing mode of /usr/bin/ansible-doc to 755
    running install_data
    error: can't copy './library/utilities': doesn't exist or not a regular file
    make: *** [install] Error 1

    Debugging 'benno' came up with this patch, which works on CentOS 6.4 and
    on Ubuntu.

check path is directory
added manpage & setup
small cleanup
shut up module_formatter in utils to avoid trace print on crud files in library

If ANSIBLE_KEEP_REMOTE_FILES=1 is set as an environment file, remote files will not be deleted, which is useful only for development debugging purposes.

Move setup.py stuff back to top level as "make sdist" was not working, plus, it makes sources cleaner.

Distutils MANIFEST.in, setup.py -> packaging/distutils/ directory.