Remove hardcoded Eclipse class output directory

I would like to propose removing the hardcoded Eclipse class output directory for a couple reasons:

* The Gradle Eclipse plugin already [sets its own default value](http://www.gradle.org/docs/current/dsl/org.gradle.plugins.ide.eclipse.model.EclipseClasspath.html#org.gradle.plugins.ide.eclipse.model.EclipseClasspath:defaultOutputDir): `${project.projectDir}/bin`, so setting it again in this plugin seems redundant
* The way it works now, the user is unable to change the location because values set in the build.gradle script are being overwritten

If you feel it is important to keep this default configuration, please consider adding it as a project configuration option so that it can be changed if desired.