Improve jiff.clone

This commit switches from using JSON.stringify + JSON.parse to a brute
force, recursive copy.  This seems to have 2 advantages:

1. It doesn't use the custom date reviver, which causes ISO date
strings to magically become Date objects *during patching*, which is
surprising.  IOW, if a patch contains an ISO date string, under
certain conditions that resulting patched object will contain a Date
object instead of a string.

2. It appears anywhere from 3-5 faster, at least on v8, than
JSON.stringify + JSON.parse.
Merge pull request #23 from cujojs/improve-clone

Improve jiff.clone