Move badges in README to top of page

This builds on #3068 to take things a bit further and move all the existing project badges to the top of the README.  You can see what it looks like at
https://github.com/eteq/astropy/tree/move-badges

I did this separately from #3068 because this is a bigger change to what people see on the github page...

Note that #3068 should be merged before this.

cc @astrofrog @cdeil @embray @mdboom