Update README with better configuration steps.

Preview [here](https://github.com/olivierlacan/factory_girl_rails/blob/patch-1/README.md)

Instead of showing something and doing the opposite, why not show the 
good default (putting the gem in both development and test groups) and 
then explain how it automatically replaces fixtures.

I also added a note about how defining RSpec as the test_framwork sets 
"spec/factories" as the default directory instead of "test/factories".

Finally, this README was not using GitHub-flavored Markdown. Now the 
code is syntax highlighted and I've also added some useful links where 
relevant. Oh, and the proper gem name as the title.

## Candidates for revision
- [x] Why is there a download section? Who download gems and wouldn't know how to download a repo on GitHub?
- [x] No Gemnasium badge?
- [x] No Code Climate badge?
- [x] No RubyGems badge?