Fix aliased fields and embedded children

When passing an :as option to SAXMachine, active_attr used the original
field name rather than the aliased one.

ActiveSupport's Hash.slice/slice! methods expect an argument list rather
than an array.

Hash.slice! returns a hash of the key/value pairs that were _removed_
rather than the modified hash itself.
Merge pull request #2 from devin-c/fixes

Fix aliased fields and embedded children