Add Backtrace for enforce

Feature/backward

* add start_record interface

* call master client in reader

* update

* add demo code in comments

* update comments

* delete unittest for recordio reader

Slice Layer

ENH: Add EigenScalar

Refactorize Scope

* Make interface of Operator to `const Scope&`

* use operator context

* optimize code

* update net infershape

* update InferShape

* disable override InferShape(scope) in OperatorBase

* change InferShapeImpl to InferShape

* add template to OperatorContext Input/Output

* merge Input InputVar, Output OutputVar

* change Inputs to MultiInput

* fix conflict

* fix MultiInput bugs and add unit test

* rename KernelContext to ExecutionContext

* clean code

* change InferShape to protected

* fix template bug

* refine code

* use InputVar instead of Input<Variable>

* typo

* optimize code

Fix bug for WarpCTCLayer.

Add PADDLE_ENFORCE for Out Of Range in `OperatorBase`'s `Input`/`Output`

fix paddle/develop build error concerning rnnop

* add master server unit test

* fix comments

* use t.Log

* fix travis can not fetch git repo

* fix git repo