Add basic connection logging

This is a start to add a logger interface to the library for debugging purposes.
This allows you to add a log.Logger instance to a client. If it is defined, it will log events related to connections.

### TODO

- [x] Log broker open and close calls
- [x] Log client broker management events

Am I missing events? Is my approach sane? Go noob here.

@eapache @bouk @fw42 @Sirupsen