[C-API] expose remote updater api

[jacquesqiao]

• In the formal work #971, @reyoung have exposed localUpdater for python, and now local training with python api works fine.
• To support cluster training in python, I will expose remoteUpdater api (and parameter server if necessary).
• I have wrote a cluster training demo for quick start using paddle binary and shell script.

[jacquesqiao]

#1018