Fixed term loading with multiple taxonomies

## The problem
When using equally named terms in different taxonomies, TimberPost::get_terms returns the the terms for **some** taxonomy, but not always the right one. Using it in a template for linking to a term archive could link to the wrong term archive.
This was due to the TimberTerm::get_post function, which just selected one taxonomy using the term ( ignoring the others ):
```
$query = $wpdb->prepare("SELECT taxonomy FROM $wpdb->term_taxonomy WHERE term_id = %d LIMIT 1", $tid);
```

## The solution
Since TimberPost::get_terms accepts a taxonomy name, it can be passed further to the TimberTerm and no guessing is needed.
With this fix, you can use
```
{% for term in post.terms('taxonomy_name') %}
	<a href="{{ term.link }}">{{ term.name }}</a>
{% endfor %}
```
in your template and get the right link.