FIX: nbviewer caching issue

fixes ContinuumIO/wakari-issues#13
DOC+LOG: Fixed logging from comments

@joelhullcio
Remove some old, unused code

This is related to issue #33 on wakari-issues.

https://github.com/ContinuumIO/wakari-issues/issues/33
Remove duplicate build #
And that, kids, is why you shouldn't edit code on GitHub

*sigh* missed that `entry_point` that was sitting under the old build.
Handle prefix hack for old versions and new
Make sure that the code is mounted at the correct prefix

We need trailing slashes in the URLs, but not when mounting the views.
Fix missing import (re: #41)

Issue #41 was caused by a 500 being encountered, which still isn't the
correct behavior, but this gets the 500 handler working correctly.
Do not allow walking the file system (fixes #3)

This avoids any possible attacks like those outlined in issue #3 where
the user sends a URL with slashes in it to load files from various
places on the server.