Check that files really present in dragging data

According to http://www.w3.org/TR/2010/WD-html5-20101019/dnd.html#dom-datatransfer-types we can check that dragged data contain files. If there are no files we should not show drop area.

My use case is as follow.
I need a drag-n-drop file upload but at the same time I need drag-n-drop of another items on the page.
At the moment during dragging of even text the drop file area is shown.
Merge pull request #155 from dmitry-dedukhin/patch-1

Check that files really present in dragging data
Updated: "ES5-shim breaking with brunch due to old version" (#154)
Merge branch 'master' of https://github.com/nervgh/angular-file-upload
Fix iframe error response handling on IE browsers

Somehow if iframe request responeded with standard 4xx or 5xx status
then iframe content is not accesible and "access error" thrown. However
on non standard error codes iframe responds 200 and correct WS response
data. Fixing this by returning `undefined` response. However since it is
WS specific code user must handle this case himself in success event
listener.
Merge pull request #159 from edzius/master

Fix iframe error response handling on IE browsers