Basic logger middleware

[CrowdHailer]

Spike of logger middleware. Basically a port of the Plug.Logger https://github.com/elixir-plug/plug/blob/master/lib/plug/logger.ex

tackles issue #73

[CrowdHailer]

Should consider making more use of the metadata feature of Logger. Ideally other middleware like Raxx.Router would add information like controller module. (By default metadata includes the module but this will only be the controller for top level code). I think we should also investigate reusing the Logger metadata for attaching contextual information to error reporting.

• Add metadata equivalent to plug_logger_json
• could add a formatter that allows setup to be $method $status https://github.com/elixir-lang/elixir/blob/v1.6.0/lib/logger/lib/logger/formatter.ex
• check appsignal, bugsnag and rollbar for their use of metadata

[CrowdHailer]

closes #73