Feature/middle level net api

Make C lib in `paddle.v2.master.client` lazy load

Add ImageExpandFunction.

Fix hl_sequence_avg_forward.

Currently, WITH_GOLANG is a compiling flag, and not all Paddle binary
enable `WITH_GOLANG=ON`. So we cannot ensure that every paddle has
`master.so`. Make load that C libary lazily could let the user run
Paddle without `WITH_GOLANG=ON`.

fix conv layer reshape

add param_attr parameter for img_conv_groups(...) in /python/trainer_config_helpers/networks.py

update gradient operator registry mechanism

Implement distributed training save model, improve master.NewClient i…

do not do log.Errorln when checkpoint is not found (which is normal)

OLD: op_type -> grad_op_creator

NEW: grad_op_type -> grad_op_creator
     op_type -> grad_op_type

Backward

Fix doc

* Refine unittest also

Replace `OperatorPtr` with `std::shared_ptr<OperatorBase>`

Remove ScopePtr and OperatorPtr