Installation instruction for development

REVIEW: I just wrote this as well as I could, but I'm no ruby expert. Needs verifying.
Add vendor/bundle to gitignore

Bundler installs dependencies to this subfolder when calling "bundle install". They should not be tracked by git.
Simplify dev install instructions by using rake install.

I'm still not sure about the many different ways of doing stuff in ruby gems. Note that sudo gem install had --development option. I'm not sure what rake install exactly does, but it is briefer.
Merge pull request #5 from najamelan/hotfix/devInstallInstructions

Installation instruction for development