Removed @parsed_configs and storing if parsed in object. Fixed leak!

[eljojo]

We were storing whether an object had been parsed or not in
@parsed_configs, inside SAXHandler, which was a huge memory leak in case
we wanted to parse really big files.

Created an instance variable in each element to store whether the
element_config was parsed already or not.

It feels a little bit dirty, but it's much better for garbage collection :)

Tests are passing.

[krasnoukhov]

Hey @eljojo, thank you for the PR.

I'm afraid that this does not make sense since sax-machine does not support such thing as streaming. So basically it can not be garbage collected until nothing references top-level document object.

In fact, sax-machine needs a lot way more refactoring to do in order to support really large files, please see my comment here.