Add numpy array support

[Themis3000]

Currently, lazy_db does not support numpy arrays in any way. Often times when working with large amounts of data, numpy is used. It would be a good idea to support numpy in order to extend the potential use case of this library.

[Themis3000]

#5 would add support for numpy arrays, but I'm leaving this issue up since tests should be done to see if there's a more space efficient way to store numpy arrays then pickle.