Better project introduction to PyPI #254

The current introduction just says:

"""Selenium2Library is a web testing library for Robot Framework that leverages the Selenium 2 (WebDriver) libraries."""

It would be great to have some more information, including installation instructions there. A very easy and DRY solution is reading the current README.rst content to long_description in setup.py. We recently did that with StatusChecker project and it works great. The same information is visible in BitBucket project pages (GitHub obviously shows READMEs same way) and on PyPI pages, and the implementation was pretty trivial.