Fix guideline issues

[acelaya]

Part of #40

This PR fixes the issues reported by the WordPress plugins team on their latest review.

• They reported an incorrect license used in the License field, and that a GPL-compatible one has to be used. I updated it from BSD to BSD-2-Clause, just in case only the identifier is wrong, as BSD appears as a GPL-compatible license.
  However, I have asked them to clarify if this is the probelm.
• I documented the integration with a third party, as we load the embed.js script from https://hypothes.is/embed.js
  This is allowed, but it needs to be explicitly noted in the readme.txt file.
  I also added links to our terms of service and privacy policy.
• I added a unique Hypothesis PHP namespace to our code, to avoid symbol name collisions with the global namespace that WordPress uses for its own internal API.

[robertknight]

I had a query about what "integrate with the Hypothesis annotations API" refers to.