[MRG] Metric documentation (mainly in classification)

I would like to partly tackle the issue in #1508. 
In this pr, I intend to add definitions to classification metrics and remaining regression metrics.

Before going into the MRG state, I still have read two or three times the added documentation to find mistakes. 

Furthermore, I am not sure about the documentation on `explained_variance_score`. It is new to me.

Questions:

1. After this pr, I would like to add some multilabels metrics, some thoughts about this?
2. Why is there a `zero_one` and `zero_one_score` instead of  `zero_one_loss` and `accuracy_score`?
3. Why `ClassifierMixin` uses a "homebrew" accuracy metrics in the score member?