Add negative subscripts support to TimeseriesGenerator and Iterator.

[OrigamiDream]

Summary

Python supports negative subscripting for all subscriptable types as standard.
TimeseriesGenerator and Iterator override __getitem__ function but does not support negative subscripts.

Related Issues

N/A

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)