log4j2 support for spring-boot

Hey,

Sorry for the big pull request. I really wanted to use log4j2's enhanced api in an upcoming project so I've provided an integration that follows the patterns for the other logging apis.

This contains:
- The logging system class in spring-boot (and an addition to the base system to pick it up)
- Dependencies on 2.0.2 in the dependency pom
- Basic config that follows the same patterns as the log4j defaults
- A starter and sample derived from the original log4j projects

Let me know what you think.

Cheers,
Daniel