Closure Compiler installation from source is out of date

[Peilonrayz]

The README is out of date as:

• Google Closure Compiler is now on GitHub (https://github.com/google/closure-compiler) rather than at code.google (https://code.google.com/p/closure-compiler/).

• The Google Closure Compiler now uses Maven (https://maven.apache.org/) rather than Ant (https://ant.apache.org/) as the build process.

  I don't know much about either Ant or Maven, but using the new repository with mvn -DskipTests worked as expected.

Also the code is out of date as:

• The build process now outputs the jar to {closurePath}/target/closure-compiler*.jar, rather than {closurePath}/build/compiler.jar. In my case the file was {closurePath}/target/closure-compiler-1.0-SNAPSHOT.jar.

  We should probably take this into consideration in setting closurePath #38