plugin XEP-0277: better resilience to broken items: - if `author` element can't be found, `publisher` attribute, then `IQ`'s `from` attributes are used as fallback to find author jid - fix categories (tags) parsing if `author` element is not found - remove items which have failed parsing from `mbGet` results (instead of using `None`).

You'll find install instructions in documentation (see doc/installation.rst).

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