implemented padding aware im2col and col2im functions
add test code to test the padding aware im2col col2im functions
add code to measure timing
remove the pad=0 case in conv_layer and im2col_layer
remove padding layers in imagenet definitions
unified to padding aware version

remove test code (no longer needed and won't compile)
remove padding_layer and its test
remove cuda_timer as is no longer needed
Merge pull request #128 from mavenlin/pad-im2col

im2col and col2im learn to pad and padding layer is obsolete