Importing Twenty Fifteen, first pass at our new default theme for 2015, set for 4.1.

It's good for posts, it's good for pages, it might be good for you. Development will occur in trunk. Have at it.

Props matt, iandstewart, iamtakashi, obenland, cainm, kristastevens, karmatosed, chellycat, lancewillett, kwight, davidakennedy. See #29799



git-svn-id: https://develop.svn.wordpress.org/trunk@29892 602fd350-edb4-49c9-b593-d223f7449a82