Add client.DeletePost()

I added a function to delete posts.

Two comments:
1. Users can only delete their own posts, so it doesn't make sense to let the function have an entity argument.  Since the entity is still part of the post url, I needed to add an entity field to the client struct.
2. I think it would be good to return the created delete post. To be in line with client.CreatePost() I didn't implement this yet. What do you think?
No need to assign this to `err`, return it directly.