Memoryerror pd sum sum

[cvanoli]

-Deleted pd.sum.sum which was giving memory error. Changed it to a for loop summing on every row

[coveralls]

Coverage increased (+0.07%) to 82.873% when pulling d390e2f on memoryerror_pd_sum_sum into c3588e4 on master.

[janowicz]

@cvanoli Thanks. Per our existing discussions with @sablanchard, this low-memory change should be made an optional one (e.g. a high-level low_memory parameter that defaults to False), so that those with the requisite memory can avoid the for loop.