Add PayloadEnclosing.setPayload(ByteSource)
Replace uses of ByteArrayPayload

Prefer ByteSourcePayload which offers a superset of its functionality.
Note that ByteArrayPayload implicitly set the contentLength while
users of ByteSourcePayload must do so explicitly.
Remove ByteArrayPayload

Callers should use ByteSourcePayload instead.
Ignore non-existent container in deleteContainer

All other blobstore providers allow this operation.  Further this
matches the behavior of TransientStorageStrategy.deleteContainer.
This commit prevents a spurious error message from
deleteContainerOrWarnIfUnable after test suite completion.
Prefer Files.delete(Path) over File.delete

The former throws an exception including the cause if the operation
fails while the latter merely returns a boolean.
Add support for Azure Copy Blob

Presently lacks support for CopyBlobOptions.  API reference:

http://msdn.microsoft.com/en-us/library/dd894037.aspx