More labels for custom taxonomies, fix #256

The reason the labels on buttons were incorrect for "Series" and "Post Prominence" was because we weren't supplying [enough labels](http://codex.wordpress.org/Function_Reference/register_taxonomy#Arguments) when calling `register_taxonomy` in `inc/taxonomies.php`. 

The default labels for hierarchical custom taxonomies are the ones for categories, which is why the button said "Add New Category".