Add steps needed to configure environment before running unit tests

It took me some time to figure out how to get a properly running environment.
I first had to understand what I needed (e.g. bundler, openssl support)
and then a simple way to install it.

The first issue that I encountered after installing bundler:
http://www.beginnerruby.com/rails-troubleshooting/fixing-opensslbundler-issue-for-rails-on-debian/

And then I had to reinstall ruby... I decided to go with the RVM approach,
which was simple enough as the installation provided feedback at different
steps to indicate how to remove conflicting packages previously installed
in Ubuntu and avoid potential errors.

There may be a need for an additional step to enable openssl support in ruby:
I did not run either `rvm pkg install openssl` before installing ruby with
rvm, neither did I provide the parameter --with-opensll-dir when installing
ruby with rvm `rvm install 1.9.3 --with-opensll-dir=$HOME/.rvm/usr` but I
assume that this was taken care of either by running `rvm install 1.9.3` or
`rvm requirements` which I have not included in these steps because it looked
from the console output like it was run by `rvm install 1.9.3` anyway.

Following discussion with Robin Dupret in the pull request, I improved
formatting and clarified the setup of the environment; I also added the
final steps with the aim of getting contributers to squash all commits
into a single one for each pull request.

I also replaced the link about topic branches; the previous link now
redirects to a generic page:
http://www.youtube.com/GitHubGuides

I replaced it with a link to "Topic Branches" subsection
in section "3.4 Git Branching - Branching Workflows"
in chapter "3. Git Branching" in the "Pro Git" book,
written by Scott Chacon.
Merge pull request #285 from eric-brechemier/contributing

Contributing: Add steps needed to configure environment before running unit tests [ci skip]