Guarantee isomorphism of flatten and unflatten commands

[sheerun]

If no changes are needed when flattening / unflattening,
the same object is returned, saving memory and allowing
for strict equality checks on results of flatten/unflatten.

This commit also adds "shallow" option for unflatten command
that ignores nested "messy" object. It is especially useful
when used with "maxDepth" option of flatten function.

For example, now following is true:

var object1 = { "foo": { "bar": { "baz": "buz" } } }
var object2 = flatten(object1, { maxDepth: 2 })
var object3 = unflatten(object2, { shallow: true })

assert(object1.foo.bar === object2['foo.bar'])
assert(object1.foo.bar === object3.foo.bar)

[timoxley]

@sheerun For API symmetry, what do you think about shallow being maxDepth, and working at the specified depth rather than just level 2?

i.e.

var opts = { maxDepth: 2 }
var object2 = unflatten(flatten(object1, opts), opts)

// or
var opts = { maxDepth: 4 }
var object4 = unflatten(flatten(object3, opts), opts)

[sheerun]

The same maxDepth won't work for isomorphic operation.

The isomorphic case is: unflatten(flatten(object1, { maxDepth: 2 }), { maxDepth: 1 })

That is for unflatten only maxDepth: 1 makes any sense, because flattening produces 1 layer from N layers (when unflattening we only want to unflatten this only layer). Am I clear?