make nbviewer a package

use tornado to serve

works enough to start committing, but still some work to do
ignore some files
move templates and static into package dir
/url[s]/url work now

GitHub blob/tree urls are redirected
py3 compat
add memcache support, and use gen.coroutine
update requirements
remove unused js, less, img
remove unused flask files
update procfile
add faq
add missing nbviewer.cache
update GitHub auth
use pylibmc

make async via ThreadPool
use thread pool for nbconvert calls
multi-file gist support
still figuring out this coroutine stuff
fix dummy cache typo
don't log auth params
add basic tests