Set @auth_key to nil when "deleting" session.
Code refactoring and added comments
Fixed initial XML request for manifest (+1 squashed commit)
Squashed commits:
[a3d77ef] added send_manifest option to upload_ovf (defaults to true).
Merge pull request #13 from frapposelli/master

Some housekeeping and added a :send_manifest option as it's not mandatory.