Improve Caffe Converter

[mli]

• Improved the documents
• Refactored the codes without changing semantic meaning
• Added convert_caffe_modelzoo.py which downloads and converts several models from caffe model zoo
• Added test_converter.py

The future roadmap:

• support multiple inputs
• it seems that there are bugs when caffe is not installed, namely only installed protobuf
• pair test with caffe. that is comparing the output between mxnet and caffe. currently we only test the accuracy of the converted model is reasonable.
• test more caffe models