Nbviewer2 speed (10 fold increase) 

On top of #41 and #42. 

The **custom cache had an issue**, meaning that it **was not used** most of the time. 
So I change for **Flask-Cache**.
This plus using **tornado as a WSGI**, and some reorganisation, made the *database* to store url acces time the bottleneck, which I did *deactivate*. 

All in all, in the good day, one dyno should now be able to respond to **~30 req/sec max**. (before it was more around 4). Minimum is still around 1.5 r/s if it has to fetch the ipynb, and grind it through nbconvert.

It is not perfect and would need some work for some modifications I plan to add, but should be a good improvement.

Apache Bench attached.

```
ab -c50 -n150 http://gistpynb.herokuapp.com/urls/raw.github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/master/Chapter1_Introduction/Chapter1_Introduction.ipynb
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking gistpynb.herokuapp.com (be patient).....done


Server Software:        TornadoServer/2.4
Server Hostname:        gistpynb.herokuapp.com
Server Port:            80

Document Path:          /urls/raw.github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/master/Chapter1_Introduction/Chapter1_Introduction.ipynb
Document Length:        299271 bytes

Concurrency Level:      50
Time taken for tests:   5.402 seconds
Complete requests:      150
Failed requests:        0
Write errors:           0
Total transferred:      44910000 bytes
HTML transferred:       44890650 bytes
Requests per second:    27.77 [#/sec] (mean)
Time per request:       1800.818 [ms] (mean)
Time per request:       36.016 [ms] (mean, across all concurrent requests)
Transfer rate:          8118.06 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:       82  238 360.3     85    1085
Processing:   738 1324 492.6   1205    4104
Waiting:      103  330 192.4    281     767
Total:        821 1562 740.8   1288    4219

Percentage of the requests served within a certain time (ms)
  50%   1288
  66%   1498
  75%   1761
  80%   1942
  90%   2694
  95%   3181
  98%   3879
  99%   4189
 100%   4219 (longest request)
```