Allowing Import of package xml

See https://github.com/joomla-projects/com_localise/issues/126#issuecomment-47503213
(Code largely inspired by the template uploadFile code)
![screen shot 2014-07-01 at 11 25 47](https://cloud.githubusercontent.com/assets/869724/3441212/c7721456-0101-11e4-89af-a4d58fb73592.png)
![screen shot 2014-07-01 at 11 34 47](https://cloud.githubusercontent.com/assets/869724/3441295/0787eb28-0103-11e4-95a8-1087f618e813.png)


Remains here to make sure the xml imported is safe.

Next step: Create an Export package xml to help sharing these.