Include custom slf4j logging appender factories in blueflood

tl;dr; We'd like a few more logging appenders.

Documentation is in the module README. 

This code used to live at https://github.com/gdusbabek/rax-dropwizard-logging-appenders, but I think bundling it with Blueflood will simplify things for users. The only downside is that a few extra jars get bundled with the project.