Logger for Saul

[danyaljj]

Closes #276

[kordjamshidi]

Why we should extend classes with logging now? We could easily import import org.slf4j.{ Logger, LoggerFactory } and use the predefined levels in there. What is the advantage of your new style?

[danyaljj]

This is a popular was of using external libraries; the pros are:

• Simplification: For a user who doesn't have much understanding of what is sl4j, LoggerFactory, etc this is simpler to just add extend Logging (just like how they add extend DataModel or extend Learnable, etc).
• Easier to use: some of the functionalities in classic logger are not trivial to do. For example, changing the logging level. The trait has easy way of doing so.
• Unification: Many desired properties can be added inside the logger trait, which will be shared across all our usages; for example you want to enforce a behavior across all logger usages in the code, it is enough to change the trait itself.

[kordjamshidi]

To me it looks just fancy :-), but ok for me to merge.

[bhargav]

Looks fine. Will merge when Semaphore run completes.