MongoDB support for storage

Hello,

I've been using blueprint for quite a while now and felt like trying out the server but I found out it only supports S3 as a backend for storage.

Using pymongo I've added support for MongoDB in the server backend as a quick personnal project.

Requirements (on top of existing ones):
====
```
sudo pip install pymongo
```

Configuration:
====
Server side /etc/blueprint.cfg
```
[server]
backend = mongodb # mongodb or s3
address = localhost:5000 # address of your blueprint server
use_https = False # indicates if we should craft url with https or not
[mongodb] 
address = localhost # your MongoDB server address
port = 27017 # your MongoDB server port
collection = blueprint_tarballs # your MongoDB collection
database = blueprint # your MongoDB database
user = myuser # your MongoDB user with access to DB (optionnal)
password = mypassword # your MongoDB user password (optionnal)
```

Client side /etc/blueprint.cfg
```
[io]
server = http://localhost:5000
secret = mysuperverylongsecret
```