Replace LOG4J12 with LOG4J2

[bbottema]

After moving to log4j12 in #14, let's modernize further and move to log4j2 as default logging implementation.

[zimmi]

Please reconsider adding a dependency on a concrete slf4j implementation. In my opinion, libraries better depend only on the api alone (and all the ones I know do so, I can provide examples if you want).

The current approach only works without configuration when the user:

• has no binding to an implementation (and is fine with the defaults of log4j2)
• is already using the same version of log4j2, in which case it doesn't help

In all other scenarios it introduces version or binding conflicts and users have to hunt down the reason.

If there is no binding to an implementation, slf4j will print a warning. If users want logging output, they should know what concrete logging system they are using and how to configure it.

[bbottema]

@zimmi I was thinking of providing documentation to exclude the log4j2 implementation from the maven dependencies so you can switch to another.

The API that is used in the code already is that of SLF4J.

However, if using the basic SLF4J implementation logging requires no further configuration from the user if they want to switch implementation, that would be preferable. I only included some implementation so that the demo and test classes produce output.

[zimmi]

What do you think about instead providing documentation to add log4j2 as the recommended modern logging implementation?

Adding a link to the slf4j manual for the demo code for users that want logging output might be enough, maybe.

[bbottema]

I added it as an optional dependency using the maven <scope>optional</scope>. This way it is included during development, but won't be used by depending projects.

[bbottema]

I also moved the log4j config file to the test resources folder now so that won't be packaged either (see #45).

[zimmi]

I agree with that decision. :)
From a user standpoint, I just want to send mail. The fewer dependencies and assumptions a library makes, the better. Opinions are for frameworks.
Thanks again for your work!