perf: speed up shallowCopy and special case Attributes cloning

`for in` is much faster than `Object.keys()` but `for in` includes properties from the prototype.

http://jsperf.com/for-in-vs-object-keys2

All the uses of shallowCopy don't deal with objects with heavy prototypes, except for Attributes instances
in $compile.

For this reason it's better to special-case Attributes constructor and make it do it's own shallow copy.
This cleans up the Attribute/$compile code as well.