Don't show the "Create Profile" link by default

Outside of WordPress.com, creating site-specific user profiles is less common of a use case. Having a "Create Profile" link just causes confusion.

However, if the user already has a mapped profile, we'd still like to show that link.
Don't show the guest author links at all in the network admin

Fixes #181
Readme update
Merge pull request #185 from Automattic/no-show-profile

Breaking change to "Create Profile" link