*Creating master_ core and master_localise packages in the component packages, separating master from user 

See https://github.com/joomla-projects/com_localise/issues/174
This PR prevents the core.xml file to show in the packages view, keeping it as a reference only.

It adds 2 master_ packages in the component packages folder with the ini files already highlighted and prevents generating and downloading installable packs from them (if someone tries, an alert will explain that the master pack should first be duplicated and customised for the language)
The localise.xml that was in the media folder is deleted.

Basically, any package created which contains "master_" in the name of the file will behave that way and can be shared, as well as the non-master ones.

Also is implemented an alert when importing an xml package to tell the user that this import may overwrite an existing file. George tried to throw that alert only when a file with same name existed but we have errors (see https://github.com/infograf768/com_localise/pull/2 )

Here is an example of what we get in the manager:
It should be improved in a next step to include a new column to show master packages.
![screen shot 2014-07-09 at 17 54 43](https://cloud.githubusercontent.com/assets/869724/3527064/6add51b6-0781-11e4-9845-c2f86759950e.png)

If you move this to /admin/packages can we remove https://github.com/joomla-projects/com_localise/blob/develop/media/com_localise/packages/core.xml ?