Taxonomy wrapper code and vocabulary code need cleanup

[argiepiano]

Since Backdrop's taxonomy vocabularies are not entities anymore (see backdrop/backdrop#520), the wrapper code for vocabularies needs to be updated. The fact that they are not entities makes it more difficult.

This is related to #71 . PR coming

[argiepiano]

PR #97

@laryn let me know if you want to take a look at these changes before I merge. To test, you can create a metadata wrapper of a taxonomy term and then access the vocabulary properties (like machine_name or description) through the chained properties, as in $term_wrapper->vocabulary->description->value().

You can also create a wrapper of a post node that has values in the Tags field, and then explore the vocabulary with $node_wrapper->field_tags[0]->vocabulary->name->value() and so forth.

Before these fixes it was impossible to access vocabulary values. This PR also takes care of the vid instances as outlined in #71 .

[laryn]

@argiepiano No, if you've tested thoroughly and feel good I say merge away. This is great -- thanks so much for all the improvements and fixes you're creating!