Cross-validation for Question-Answering

[bogdankostic]

This PR adds cross validation for Question Answering.
It is especially useful for evaluating small QA Domain datasets with long documents.

Features:

• Optimally balanced document wise splitting of folds (realistic splits)
• Down sampling of negative examples in train set (useful for long documents)
• New recall @ n metric that uses multiple predictions

[Timoeller]

Please add the test for tensor indices.
Apart from that it looks very good.