Prepare for python3

Hello, 

I am trying to port zipline to Python 3. This patch only prepare is for Python 3. So that after 2to3, zipline is ready to go.

So for now, It looks like some of the str need to force encoding, as well as a few unit test fixes. Also need to add comparision between class Transaction(), Event() and Order()

I tested this with Python2, all unit tests are passing.

Currently, there is one issue in Python3 with Requests https://github.com/kennethreitz/requests/issues/1434
After that change, and one more change for itervalue, zipline will be Python 3 compatible.