Feature suggestion: support logger handler

[crbelaus]

The error tracker does not currently provide a logger handler that can catch and report unhandled errors. As shown in #128 this is one of the main reasons that make people use Tower as an ErrorTracker wrapper.

I believe that we could provide a native logger handler and thus eliminate this need for a third party wrapper for specific cases.

[odarriba]

We are going to prioritizee this one, as it is a reason for many people to use other wrapping libraries and that leads to other unwanted bugs.