Add SBT options panel controlling console arguments

This pull request adds an SBT panel to Preferences > Miscellaneous category. Currently, the panel supports configuring initial and maximum heap size, maximum PermGen size, and a space for entering additional arguments. I believe that the defaults are the same as the previously hardcoded values. The screen shot attached shows the settings that I'm currently using. I made this change because projects that I'm working on need SBT to have more memory available to it. Although I could recompile the settings, I thought it would be nice to have them configurable.

I copied getSbtArgs out of core ScalaExecution because I didn't want to push the configuration panel into core. I think the SBT-specific stuff should be contained in the plugin that adds SBT support.

I also left SbtPanel as a Java class. This makes it easy to modify the panel layout using NetBeans UI designer. It could be converted to Scala at the expense of making it harder to modify later.

Let me know if you think any changes should be made.

![screen shot 2014-03-16 at 10 01 57 pm](https://f.cloud.github.com/assets/8850/2432892/6decf8f0-ad79-11e3-92ce-cd9f28d8a474.png)