Support Java 6

Current plugin is compiled against Java 7, and if you try to run it under Java 6 it fails with:

```
java.lang.UnsupportedClassVersionError: iron9light/coffeescriptMavenPlugin/CoffeeScriptWatchMojo : Unsupported major.minor version 51.0
```

Is it possible to recompile it with Java 1.6?
