1. Add switch function for switching image dimensions order
2. Add CpuMatrix::backwardSoftmax function
3. Add pixel softmax layer, python wrapper and grad_test

* We should not EXPECT_EQ between a float value and a int value.
  Use ASSERT_NEAR instead.

No longer need to specify target_inlinks or is_seq.

No longer need to use SubsequenceInput. The framework will detect.