Save remote document to store after downloading

When a document's source is specified as an URL it was downloaded but
then never saved to the (file|s3) store.  The import action then would
error out later when it attempted to read the meta data from the
original file.
Add p3p header to DocumentsController#show

Move the set_p3p_header method from authentications controller to a
protected method on the application controller so it can be shared
between all controllers who may be embedded in an iframe.

This should provide a solution for issue #39
Merge pull request #52 from nathanstitt/feature2

Save remote document to store after downloading
Merge pull request #53 from nathanstitt/feature3

Add p3p header to DocumentsController#show
Update worker ami
Increasing the warning queue length.
Re-enable small document imports on express node.
Merge branch 'master' of https://github.com/documentcloud/documentcloud into accounts_json_include_pending