Replaces railroady with rails-erd for ERD generation

The following commands will produce `doc/erd_basic.pdf` and `doc/erd_full.pdf`:
```
 bundle exec rake erd notation=bachman polymorphism=true inheritance=true attributes=false filename=doc/erd_basic
```
```
bundle exec rake erd notation=bachman polymorphism=true inheritance=true filename=doc/erd_full
```

Information on the ERD gem can be found at:
- http://rails-erd.rubyforge.org/
- http://rails-erd.rubyforge.org/customise.html