add timeout configuration.

[Ankush-Chander]

• [x] Allow user to pass timeout value
• [x] All test passed
• [ ] Documentation

I have added a timeout parameter in the requests so that user can set it based on his own requirement instead of relying on default values.

Example usage:

works = Works(timeout=30)
w1 = works.doi("10.1007/BF01827854")

I believe this will resolve #31 Please review.