[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
Agreed on the separate JAR, but I do not know if the second JAR is then resolvable from maven.  I believe the newer libraries already include the ant task as I just wrote one the other day with one of the JAR's from maven.  Maybe I am mistaken and my maven cache mis-lead me but I believe that #121 is a non-issue.  Either way, I also think that the current implementation resolves that problem.

In my opinion, the second JAR is not needed and everything can be packaged in the original.  When your application goes to resolve the dependencies, its going to pull down the same JARs anyway.  The major benefit that we get out of this code change is that the JAR is executable without needing to download the repository and build the executable JAR with ant.