Improve 'Read later' to not require a window

[Rami114]

Right now the 'Read later' feature requires a new window to be opened, which will close if the user is actually logged-in to Pinboard.in. Otherwise, it redirects to the login page and remains open.

Possible options:

• Ask Pinboard to allow jump=close to work for non-logged in users (but silent failure?)
• Use an XHTTPRequest and confirm success by checking for the login redirect
• Make the add-on aware of user status on Pinboard (background poll?)

The status quo reflects what the other add-on did but I'm not happy with it.

[Rami114]

Now that API support is in thanks to #3 we can allow users who add an API key to use the underlying API call. That will mean no window for API users!