Enhanced installation instructions to README.rst

This commit mainly contains better installation instructions (#255).
Additionally 'Robot Framework' was made a link and trailing whitespace
removed from few places.

After writing the new instructions, I noticed that there already were
better instructions in INSTALL.rst. I decided to commit my changes anyway
because they contain better pip examples and explain proxy configuration.
Instructions in INSTALL.rst were also partly outdated.

Having these new instructions and old instructions in INSTALL.rst obviously
doesn't make any sense. We need discuss do we want to keep more detailed
instructions in README or have separate INSTALL. Let's continue that
discussion in issue #255.