New TaxonomySpec class to pass to create_taxonomy().

To create resources we use `specs`, for example, the method
```
tamr_unify_client.categorization.category.collection.CategoryCollection.create()
```
takes as input a `CategorySpec` class to create a new category in a taxonomy.

To create a taxonomy in a categorization project we have the method
```
tamr_unify_client.categorization.project.CategorizationProject.create_taxonomy()
```
However, no `TaxonomySpec` class exists.

To be consistent, it would be nice to have a `TaxonomySpec` class.