React port

@aligreen 

Could use some more vigorous testing, but seems to work well for me.  Added in this port:

- React / JSX: There's now a folder called `src/` which contains jsx syntax, which is the html looking stuff in the `apod.js` file. To run the extension, this needs to be compiled to a 'build/' folder. I added README instructions on how to do that. When we upload the zip to the chrome web store we will include the compiled `build/` folder, even though it's ignored in git. 
- black gradient so you can read the title a bit better on white backgrounds
- show or hide a settings pane
- stretch or center the image using slider in settings
- save all your long term settings (military time, image preference, etc) in localStorage (we can easily save anything we want in this way, if you can think of something else that would be nice to have persistent in your new tabs (bookmarks, top sites whatever) )