Add `iter` parameter

Train for `iter` epochs
Fix timings
Remove line-ending whitespace
Relax test requirements

Travis CI previously failed (only for Python 3.4)
Revert "Relax test requirements"

This reverts commit 7fb86513547bec5d120a310aab24e8c7ce3272de.
Use corpus repetition instead

Added a new class in utils to repeat a `corpus` n times.

We could use `RepeatCorpus`, but we would need to compute the number of
sentences beforehand.
No need for self.iter
Correct total_words
Reduce memory requirements
Merge pull request #243 from sebastien-j/iter

Add `iter` parameter