DOC improvements

* add a more prominent warning for urljoin_rfc;
* mention Python 3.x support in README;
* move parts of README to the documentation;
* undocument deprecated w3lib.form module;
* enable "viewcode" Sphinx extension;
* make docs self-contained (by duplicating some of the info from README);
* remove unneeded modules.rst file;
* remove a link to unmaintained NEWS file.