New functionnality to let add files in Core package

As we have no specific editor for the xx-XX.localise.php (site and admin) or the xx-XX.css file (admin), this lets users upload these files where they should be in the site language folders before creating the final Core package.
The new button "Upload specific files" is visible when one edits a core package.


![core_package_edit](https://cloud.githubusercontent.com/assets/869724/4032925/642d0e60-2c75-11e4-9724-43d1b3f42fa3.png)


The Modal lets choose the location and browse to upload.
![fileuplaodmodal](https://cloud.githubusercontent.com/assets/869724/4032942/819ff37c-2c75-11e4-8e26-62f0302edbba.png)

The code limits the files types to .ini, xx-XX.css, and xx-XX.localise.php