Fix summing of losses

[agude]

total_loss was being added to itself at least once in the sum, now it is excluded.

Example output from before the fix:

iter 0: mid_box_reg_loss: 0.001, captioning_loss: 50.864, end_objectness_loss: 0.087, mid_objectness_loss: 0.140, end_box_reg_loss: 0.003,  [total: 102.187]

And after:

iter 0: mid_box_reg_loss: 0.001, captioning_loss: 50.864, end_objectness_loss: 0.087, mid_objectness_loss: 0.140, end_box_reg_loss: 0.003,  [total: 51.095]

Fixes #32.

[jcjohnson]

Thanks!