Enhanced giraph-debug to give better error msg

when a bogus mode is passed, e.g., `./giraph-debug hui` instead of `gui`.
Enhanced giraph-debug script not to fail even when no .jar is present

and use target/classes directory instead
Polished pom.xml to build under Giraph trunk
Adhering to giraph's POM style
Cleaned up gitignore
giraph-debug now always injects necessary classes to the given jar

so that it can be used on foreign user jars (as long as the user does not
have conflicting classes with giraph-debugger's).
Merge pull request #46 from netj/giraph-module

Getting ready to be submitted as a Giraph module