Use HTTP/1.1 for TensorBoard

This change causes less network and GIL thrash.

Previously, a separate TCP connection and non-pooled thread had to be
created for each and every request. By using HTTP/1.1, each connection
and thread can serve >1 requests.

This change might not necessarily cause elements on the page to render
more quickly due to the way browsers limit simultaneous connections per
hostname.

The following is Chrome network inspector for TensorBoard on localhost:

Before: http://i.imgur.com/tjLJWGv.png (refresh of images page)
        http://i.imgur.com/ushQpCg.png (each fetch from localhost)
After:  http://i.imgur.com/B1nZXX2.png (refresh of images page)
        http://i.imgur.com/6ile1Ge.png (each fetch from localhost)
Change: 130325620