Fix im2col nonsquare bug (#284)

Commit 58fe797 modifies the tests in test_convolution_layer.cpp to expose the im2col GPU bounds checking bug for non-square inputs identified by @kmatzen (#284).  Commit 1fa5ae2 fixes the bug and makes the tests pass.  The change to im2col.cu can be cherry-picked into master but the tests won't work on master without modifications (due to using convolution_param()).