JCLOUDS-402: Allow Guava MediaType Content-Type

This promotes type-safety.  Keep the String interface for unknown
content types.
Remove BlobBuilder and Payloads.calculateMD5

Callers should instead explicitly set contentMD5, usually with the
results from Guava Hashing.md5(). This narrows the API and removes a
strange IOException from callers.  Further it removes a dangerous
rebuffering of arbitrarily-large non-repeatable Payloads.
JCLOUDS-471: Adds pool field to FloatingIP and updated related tests.
Updated FloatingIP test and related input data