Limiting language choice when creating packages

Limiting language choice to existing core site and administrator xx-XX.xml when creating packages. Cleaning up 'installation' related code for package.php.

Before:
![screen shot 2014-06-21 at 07 10 39](https://cloud.githubusercontent.com/assets/869724/3347891/7728d158-f902-11e3-8fa1-8b0e6cd46086.png)


After
![screen shot 2014-06-21 at 07 08 25](https://cloud.githubusercontent.com/assets/869724/3347890/3251a10e-f902-11e3-8f87-b72f74ec97d7.png)

I think this can be a JFormFieldList instead and do a getOptions instead of doing a getInput but looks okay, so merging and creating an issue for future