Improve performance of ndb_json module

[erichiggins]

Benchmark against stock json module on Python 2.7:

Running json benchmarks...
Running ndb_json benchmarks...

Results
=======
loads (large obj)
-----------------
json            1.103s
ndb_json        2.200s

dumps (large obj)
-----------------
json            0.559s
ndb_json        0.553s

loads (small objs)
------------------
json            1.201s
ndb_json        2.760s

dumps (small objs)
------------------
json            0.913s
ndb_json        1.253s