Fix #29: Make maxStackSize configurable

[longwa]

Allows configuring the maxStackSize on the NewRelicEncoder for logback.

[CLAassistant]

All committers have signed the CLA.

[jeffalder]

This is great, thanks for the contribution! Is this settable via XML as-is? If not, what would it take?

[breedx-nr]

Thanks for the submission! This is a great idea.
Overall, it does look good, but I did have a few small suggestions and a question.

[longwa]

This is great, thanks for the contribution! Is this settable via XML as-is? If not, what would it take?

I'll create a simple test project in Java using XML and test it shortly. Thanks for considering the change.

[breedx-nr]

Thanks for the help!

[cortexcompiler]

Does this mean that the README could be updated to show that this is supported? :

        <encoder class="com.newrelic.logging.logback.NewRelicEncoder">
        	<maxStackSize>50</maxStackSize>
        </encoder>