Add Traceback support to thrown Exceptions

[addyess]

I wanted to be able to see the traceback of where the promise was rejected during a p.get()

• This maintains the traceback if there was a thrown Exception in the reason.
• On p.get(), I added a raise with the original traceback to see where the ACTUAL traceback occurred -- not the traceback of the p.get()
• added tests to demonstrate the addition of a 'traceback' attribute to the reason exception

I didn't see instructions on how to run the tests so i updated setup.py to run pytest with python setup.py test

[coveralls]

Coverage decreased (-2.9%) to 97.083% when pulling 8f8306d on addyess:master into b4ec8f1 on syrusakbary:master.

[coveralls]

Coverage decreased (-2.9%) to 97.083% when pulling 8f8306d on addyess:master into b4ec8f1 on syrusakbary:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0ad0b59 on addyess:master into b4ec8f1 on syrusakbary:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0ad0b59 on addyess:master into b4ec8f1 on syrusakbary:master.

[coveralls]

Coverage remained the same at 100.0% when pulling d1d1522 on addyess:master into b4ec8f1 on syrusakbary:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 5148371 on addyess:master into b4ec8f1 on syrusakbary:master.

[tlmurphy]

@syrusakbary Can this be merged? Seems valuable.

[syrusakbary]

Closing in favor of #25