WebAgg: extracted figure_div style into css and changed layout

Set #figure-div display style to inline-block to use free space on the page
more effectively. See http://www.vanseodesign.com/blog/demo/inline-block/.
Additionally, set margin to 10px around every figure, so padding-bottom
becomes unnecessary.
WebAgg: reuse already running Tornado ioloop

    If a Tornado ioloop is already running before WebAggApplication.start() is called,
    it should be re-used instead of trying to start it again. This allows one to
    use WebAgg in his/her external Tornado application.
Merge pull request #2825 from vmarkovtsev/master

WebAgg: extracted figure_div style into css and changed layout