write some default unit tests for recommenders

[zenogantner]

E.g.:

1. check the ToString() method output for commas
2. check whether ToString() returns the correct class name.
3. check SaveModel() -> LoadModel() behavior
4. check whether SaveModel() stores the correct class name.
5. check convergence behavior on training data for iterative methods (with regularization turned off)
6. check robustness for unknown user/item inputs

[zenogantner]

(1) and (2) is done and will be released with version 3.05.