Improve speed of on-disk json key access

[chrisjsewell]

json_keys: ijson.parse is really slow, maybe do partial in_memory, using ijson.items
or, if implment json lazy loader ( #4 ), use that

     %timeit json_to_dict('test.json',['initial','crystallographic'], in_memory=True)
     100 loops, best of 3: 5.1 ms per loop
     %timeit json_to_dict('test.json',['initial','crystallographic'], in_memory=False)
     100 loops, best of 3: 4.78 ms per loop
     %timeit json_keys('test.json',['initial','crystallographic'], in_memory=True)
     100 loops, best of 3: 10.7 ms per loop
     %timeit json_keys('test.json',['initial','crystallographic'], in_memory=False)
     1 loop, best of 3: 697 ms per loop