MongoDB Backend

    $ mongod
    $ ipython
```Python
In [1]: L = [(1, 'Alice', 100),
   ...:  (2, 'Bob', -200),
   ...:  (3, 'Charlie', 300),
   ...:  (4, 'Denis', 400),
   ...:  (5, 'Edith', -500)]

In [2]: from blaze import *

In [3]: import pymongo

In [4]: db = pymongo.MongoClient().mydb

In [6]: coll = into(db.mycollection, L, columns=['id', 'name', 'amount'])

In [7]: t = Table(coll)  # wrap up with Blaze

In [8]: t
Out[8]: 
   amount  id     name
0     100   1    Alice
1    -200   2      Bob
2     300   3  Charlie
3     400   4    Denis
4    -500   5    Edith

In [9]: list(coll.find({'name': 'Alice'})) # MongoDB query
Out[9]: 
[{u'_id': ObjectId('53dc14e2f047051b9e4d7c0a'),
  u'amount': 100,
  u'id': 1,
  u'name': u'Alice'}]

In [10]: t[t.name == 'Alice']  # Blaze Query
Out[10]: 
   amount  id   name
0     100   1  Alice
```

The backend supports Projections, Selections, some reductions, some split-apply-combine
This could use a docstring here indicating the class's purpose.