Making the extra-spec handling in Volume Types more robust.

Was a little presumptive before, now it has explicit checks for
things existing.
Merge pull request #28 from JohnPostlethwait/type-fixes

Making the extra-spec handling in Volume Types more robust.
Clean up volume type create method

The creat method should accept a well-formatted object of key
value pairs for extra_specs, not some form-encoded-esque thing
that must be parsed.