second attempt at handling missing mf2 properties

the first attempt, dd98445c7f53c9572e8b30d96d1e224c07d320a9, was flaky and half broken. the microformats2 gem's dynamic objects don't always handle respond_to? for dynamic mf2 property methods. this approach is dumber; it just tries to call each method and catch NoMethodError if it's raised.

i don't fully understand what's going on with microformats2's dynamic objects, beyond the fact that they don't use method_missing. i just know that this works much better.
Merge pull request #32 from snarfed/bug-fix-take-2

second attempt at handling missing mf2 properties