Support decorator syntax for add_conversion_method

[nstarman]

It would be very convenient to be able to do

@add_conversion_method(type_from=...., type_to=...)
def conversion_function(obj):
    ...

[wesselb]

Hey @nstarman! I believe this is what your PR contributed, right? :) I'll therefore close this issue.

If you meant something else, please reopen the issue!