BUG: Use NullHandler to scope user algorithm code logging

[abepolk]

As I mentioned in my comment on the issue, I have used NullHandler to block logs from the algorithm from being sent to stdout or stderr by pushing the NullHandler a context handler just outside the places where the algorithm and its methods (i.e. initialize) are invoked. Because the NullHandler lies in a higher position on the stack than any other handler called internally to zipline, it handles logs that are not handled by the algorithm. The unlog_method scopes a passed method by a context handler containing NullHandler. Resolves issue #2296.