add TaskFail interface

Fix slow parsing a recursive depends topology

* generic.cmake can propogate dependencies through libraries. It is no
  need to specific all dependencies.

* Fix  #2797
* It because trainer_config_helpers' __dfs_travel__ did not record the
  node which travelled, and if the topology has a recursive dependency,
  there are some nodes will be travelled multiple times.
* Add a `travelled` set to record which node is travelled.
* Also add a unittest for this situation.

update docs url

fix dynload link error: undefined reference dlsym

Network implement

Pserver Save state

update the md5sum of WMT14 trained model.

* add device_context

* add unittest for device_context

* transfer to use function paddle::platform::throw_on_error

* fix cuda build error

* using dynload functions

* follow comments

"lr state serialization"

Fix paddle build when it's a subdirectory of another project

Update comments of maxout_layer

mistaken: Folk -> Fork in develop branch