Closes #11 - Added `preserveDescriptors` option.

By default now, property descriptors are *not* preserved when cloning an object.
This decision was made so that the implementation would adhere more to the
original structured cloning algorithm spec, thus being more predictable for
developers. However, this can now be enabled via a `preserveDescriptors` option.
Merge pull request #13 from traviskaufman/v1.1

Closes #11 - Added `preserveDescriptors` option.