Add TrainingDataSet model.

I also set up Rails association between Developer and
TrainingDataSet model.
Add Algorithm model.

With this, I also seed the database with our first algorithm: the simple
linear regression.
Add validations at model layer.

* Add validations to TrainingDataSet.
* Add validation to Algorithm.
Add model unit tests.

* Add specs for TrainingDataSet.
* Add specs for Algorithm.
* Add factories.rb to define model directives.