JCLOUDS-457: BlobStore MultiPartUpload strategy

The code related to the MultiPartUpload strategy has been added.
MultiPart uploads use an upload strategy (e.g. sequential vs parallel)
and also a slicing strategy to split the payload in different parts.
We have not yet used this in jclouds, but you might want to look at [Google auto](https://github.com/google/auto) for these simple classes.