Refactor s3

Allow any S3 compliant service through the S3 adapter.  Refactor the code to use boto's url generator for the signed service requests.  This also incorporates the changes from pull request #367 for deleting partial uploads.

A new optional parameter was added for creating an S3 assetstore: service, which is of the form [http[s]://](service host)[:(port)].  This is added to the admin console as well.
We should use this for screenshots and all the other stuff we currently pipe to the console. But we can do that on another topic.