Generator support

[fcracker79]

The pull request include support for repeatable generator functions, that is, generator functions that can be invoked many times.
I have implemented 2 different strategies:

1. Full fetch of data: this is useful if you cannot preserve a deterministic order of the generated items; works only if the generator does not produce many elements
2. Real generation of data: this works only on deterministic generators, that is, the order of the generated elements are always the same. Works pretty well with very long generations of items

I have also added Python 3.6 to the travis environments.