[feature/plupload/integration] Integration of Plupload

This commit is a highly-refactored and up-to-date version of Fyorl's work
which was part of his Google Summer of Code 2012 project "Attachment
Improvements".

PHPBB3-10929
[feature/plupload/integration] Add missing javascript file.

PHPBB3-10929
Merge pull request #1772 from bantu/feature/plupload/integration

[ticket/10929] Integration of Plupload