Fix nasty Safari bug with uuids

Safari (as of Version 6.1 8537.71) has a bug related to ES5
property values. In some situations, even when the uuid has already
be defined as a property value, accessing the uuid of an object can
make it go through the defaultUuidGet like if the property descriptor
was still the original one. When that happens a new uuid is created
for that object. To avoid that we always make sure that the object
have a _uuid that will be looked up to at defaultUuidGet() before
generating a new one. This mechanism was created to work around an
issue with Safari that didn't allow redefining property descriptors
in DOM elements.