support 'in' and 'not in' operator

[reverbc]

Adding support for the in and not in operator by implementing __contains__() method as the following example:

>>> 'child' in untangle.parse('<root/>').root
... False
>>> 'child' in untangle.parse('<root><child>value</child></root>').root
... True

This could be useful when dealing with dynamic xml API response.

[coveralls]

Coverage increased (+0.3%) to 35.577% when pulling 63baee5 on reverbc:support-membership-operator into 2119763 on stchris:master.

[stchris]

Very nice! Can you add yourself to the AUTHORS file as well?

[reverbc]

@stchris Just did. Thanks for accepting the PRs 🎉

[coveralls]

Coverage increased (+0.2%) to 35.514% when pulling 87fb7f1 on reverbc:support-membership-operator into 2119763 on stchris:master.