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.
Check this chunk of code added to BaseSlicingStrategy:

@Inject(optional = true)
@Named("jclouds.mpu.part.ratio")
private final double ratio = DEFAULT_RATIO;

Aws-s3 injects the part size in a similar way. What do you think?

Note that ratio is the quotient of the part size divided by the number of parts, so the bigger the ratio the less parts we get. In addition, we could offer more implementations of the slicer, also the user could make its own implementation to get the desired results.