invalidate s3 url cache upon storing file
this prevents future requests to the same s3 bucket/key to get the same s3 url that would hit the browser cache instead of retrieving the item anew
Merge pull request #34 from talentral/master

invalidate s3 url cache upon storing file
don't override placeholder attribute if one was already manually set
Merge pull request #35 from talentral/master

don't override placeholder attribute if one was already manually set
make forms closer to HTML specs by setting field attributes, etc
don't override required attribute for rendering the form initially