Allow false for hash maps

Well, none of Babelicious is set up to handle objects in hashes. Nor could I think of a reason why.

If we're mapping xml to a hash, or another xml. Xml is basically a string, so no need for objects.
If we're mapping hash to hash, or hash to xml. I guess you could have objects for keys, but that makes path matching weird.

For instance, let's say you have this standard hash:
```ruby
  { :contact => { :address => { :street => "1232 N." }}}
```
And you want to map that hash to:
```ruby
  { :street_address => "1232 N."
```

Then in babelicious you would have:
```ruby
  m.map(:from => "contact/address/street", :to => "street_address")
```

Now to do the same with objects, it gets weird.
```ruby
  m.map(:from => Contact.new?/Address.new?, :to => "street_address")
  m.map(:from => TypeOf(Contact)/TypeOf(Address)
```