Fix _really_ nasty typo

It was especially nasty, because, half of the time, the result is what
you'd expect.
Prevent ws_Statement_deinit from deinit'ing *NULL
Add support for implicit arguments

The idea is that you can pass a number like so:

    { "add": 4 }

Which adds the four last numbers on the stack.
Merge pull request #342 from matthiasbeyer/add-support_for_implicit_arguments

Support for implicit arguments