Updates Plupload to version 1.5.4.

The JavaScript, Silverlight, and SWF files had 
changes. The stylesheets did not have any changes.

Plupload release notes:
  1.5.4: http://j.mp/Plupload154
  1.5.3: http://j.mp/Plupload153
Updates Plupload to version 1.5.7

http://www.plupload.com/punbb/viewtopic.php?id=3711
Merge pull request #21 from matiaskorhonen/1.5.7

Updates Plupload to 1.5.7