Add installation instructions to the README

On copying the documentation from heroku I came accross installation instructions that seem to have gotten lost when the project came to github. I felt it would be useful to reintegrate them in the README. I don't know if there was a motivation for not bringing them in from heroku.
add uninstall instructions also

I'm not a veteran ruby expert, I don't know if this suffices.
Added ruby-dev to requirements in readme

For the gem install --dev option
Merge branch 'master' into hotfix/readmeInstallInstructions

* master:
  Add development group to Gemfile
  Improve work with requirements. Remove circular dependency and some "require rubygems"
  Update website and meta in the readme
  Add documentation and .gem file to .gitignore
Merge pull request #1 from najamelan/hotfix/readmeInstallInstructions

Add installation instructions to the README