Update the examples for current version of Stream, simplify example, and add more docs

[benlk]

What this does

• Updates the example connector to support Stream version 3.x: new namespace, make things not-static
• Removes the post-generator example code, and simplifies the example connector. The example connector now logs when odd-numbered posts are edited.
• Adds large amounts of in-code documentation to explain the "magic" parts of the Connector class.
• Expands the readme with images and explanations of the concepts that are reflected in the code

Things you may want to edit before merging

• The author, author URI, and contributors sections of readme.txt, readme.md, and stream-connector-example.php: I added credits for me and for 10up. 10up is my employer and subsidized this work.

Other recommendations

• Add a mention of the ability to write custom Connectors to the Stream plugin readme, so that it shows up on https://wordpress.org/plugins/stream/ :

  For developers, Stream supports third-party custom connectors. To build your own custom connector, see the custom connector example code.

[jeffpaul]

@lukecarbis @ccmoralesj any chance of getting this through review to help update the example code in this repo?

[lukecarbis]

lgtm!