Adding Mining the Social Web to the frontpage.
Merge pull request #142 from ellisonbg/socialweb

Adding Mining the Social Web to the frontpage.
fix typo in mining the web url
log more information about failed requests
add nbviewer git info to footer
get the proxied IP in request logs

instead of logging the load-balancer IP
handle more cases in failed upstream fetch

- turn 599 (tornado client) errors into 400, 404, not 502
- turn 4XX in 400, not 500
move log_request to nbviewer.log
Explicit 404 on base64 data-uri requests

seem to come from super-old browsers
hash urls as key to memcache

there is a limit to the length of a key in memcache
tornado already supports xheaders, just needed to turn it on.
Quote URLs. Fixes #145.
Encode URLs for the GistHandler
Merge pull request #143 from minrk/logger

Some more logging changes
Merge pull request #146 from rgbkrk/escape_in_dir

Quote URLs. Fixes #145.
fix commit url in template
Merge pull request #144 from minrk/cachekey

hash urls as key to memcache
self.short_url typo
turn any exception rendering a notebook into 400

not just NbFormatError

And only show tracebacks on the non-nbformat errors.
Merge pull request #147 from minrk/invalid-notebook

turn any exception rendering a notebook into 400