fix choice of clang++ in OS X build
[example] elaborate net surgery description
Fix Makefile warning error message.
use layer_param instead of layers_[layer_id]->layer_param()
Add param_propagate_down_ vector to layer, populate according to
blobs_lr in Net::Init
Make ConvolutionLayer and InnerProductLayer abide by param_propagate_down_
Use Blobs instead of SyncedMemorys for the bias_multiplier_'s.
Add Net Test to verify correct param_propagate_down behavior.