[AUTOTVM] Improve tutorial and logging (#1544)

python/tvm/autotvm/measure/__init__.py

python/tvm/autotvm/measure/measure_methods.py

python/tvm/autotvm/record.py

python/tvm/autotvm/task/dispatcher.py

python/tvm/autotvm/tophub.py

python/tvm/autotvm/tuner/callback.py

python/tvm/autotvm/tuner/sa_model_optimizer.py

python/tvm/autotvm/tuner/tuner.py

python/tvm/autotvm/tuner/xgboost_cost_model.py

python/tvm/autotvm/util.py

python/tvm/rpc/base.py

python/tvm/rpc/proxy.py

python/tvm/rpc/server.py

python/tvm/rpc/tracker.py

tutorials/autotvm/tune_conv2d_cuda.py

tutorials/autotvm/tune_nnvm_arm.py

tutorials/autotvm/tune_simple_template.py