[WIP] Metric classification documentation

I would like to tackle partly the issue in #1508. 
In this pr, I intend to add definitions to regression and classification metrics.

Before going into the MRG state, I still have read two or three times the added to doc to find mistakes. 

Furthermore, I am not sure about the documentation on `explained_variance_score`. It is a new to me.

Questions(s):

1. After this pr, I would like to add some multilabels metrics, some thought 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?