Added more configuration options

First of all I want to thank you for making this great plugin. I was using the plugin to add compass to a project and ran into a few problems when setting it up with an old Java version. The following are the changes I made:

1. Custom JVM Args: The previous version of this plugin had three custom jvm args meant to improve performance. Unfortunately these options do not work on all versions of the jvm (I had issues running them with an older version of the JVM). Instead, I added the ability to specify jvmArgs in the standard configuration option 

2. Specify Gem Versions: The previous version of this plugin allows you to specify additional plugins you would like to use, but they don't allow you to choose a version. I added the ability to specify a version using the following format "gemname:version". This allowed me to create this new feature without breaking backwards compatibility.
Note: As part of this change I had to move to calling the `gem install` once per gem to be installed. This causes the gem installation to be much slower; however, this is the only way to multiple gems with a specific version.

3. Specify JRuby Version: I added the ability override the default jruby version using the standard configuration rather than having to use a resolutionStrategy to force the dependency

I also noticed that several of your projects are licensed under Apache 2 so went ahead and added the Apache License file to the project as well.

Thanks!