[Issue #450] Fixed the publishing of the jar to include the Main-Class entry needed for execution

Added the configuration needed to fix the builds that are published to maven.  Currently, the MANIFEST.MF file does not contain a Main-Class entry needed for execution.  The commit below fixes that and adds the line to the manifest file.

**Verification Instructions**
* Perform a *mvn clean package* to generate a new version
* Un-jar the generated file
* Inspect META-INF/MANIFEST.MF to ensure that the file now contains a Main-Class entry.
* Ensure that the jar is executable

**Existing Issues**
https://github.com/google/closure-compiler/issues/450
https://github.com/google/closure-compiler/issues/227
Ideally, there probably _should_ be a second jar with everything bundled inside (including the Ant task), as that would resolve https://github.com/google/closure-compiler/issues/121 .

If this is a favorable change and you do not have time to tend to it, please let me know and I will fork and open a pull request.