Add a main class

Did a few things:

- Added a main class that can be used to run the validator standalone; spits out a human-readable report to the console (the report is also valid Markdown so can be used with a web server)
- Added a logging framework (java.util.logging) rather than System.out.println, so that log messages can be controlled by the user
- Added source and target to the pom so that command-line builds are possible (both currently set to Java 1.6 as that is more or less the current least common denominator)
- Added the shade plugin to the pom to create a standalone JAR that can be used to run the command-line validator