Adds new file only if before_add was successful

With this change before_add can be used to do javascript validation.
After an invalid file was not uploaded the next valid upload will 
trigger 's3_uploads_complete' and not only 's3_upload_complete'
Merge branch 'refs/heads/master' into patch-1

Conflicts:
	app/assets/javascripts/s3_direct_upload.js.coffee
Merge pull request #65 from mitnal/patch-1

Adds new file only if before_add was successful