optimize readme and meta descriptions

[pfefferle]

Some readme cleanups:

• plugin header uses the readme description now (without Facebook and Twitter)
• better tags
• fixed some typos and fb:admins in the FAQ
• new FAQ entries for the image handling and Jetpack
• fixed outdated links (Graph API, issue look into using get_the_title() #17), the Like Button is gone, so it links to the Share Button now