Correctly apply padding to pooling layers when upgrading from V0

The pooling layer supports padding for average and max pooling, however the V0 prototext upgrade script doesn't correctly translate pooling layers with padding when upgrading. This fixes that bug. 

Currently there is no check that the pooling layer being upgraded uses average or max pooling. I figure it's best to translate the prototxt as is and let downstream operations catch that error (also, in the future padding may be supported for other pooling operations).