Feature/caffe importer improvements

[sbodenstein]

A few improvements:

• allows importing from prototxt definitions with Input layers (currently doesn't support this, and is most common case)
• add support for TanH and Sigmoid activations (needed by some Caffe model zoo nets, eg "Holistically-Nested Edge Detection")
• a mean image importer (import caffe mean image protobuf to NDArray)
• Some comments on the current incompatibility between the strided pooling output dims of MXNet and Caffe that is not dealt with by the importer (can be frustrating for users of the importer not to know about this issue).