Add train function in NeuralNetwork Class.

Add train function in NeuralNetwork Class.

Add train member function of NeuralNetwork Class.
- NeuralNetwork Class has DataBuffer instance to handle datas.
- New Keywords are introduced for specify data set
  . TrainData, ValidData, TestData, LabelData
- DataBuffer instance is initialized with parameters from Neural
Network instance.
- In train function,
   . Data Buffer instance is running ( collecting or generating data )
   . get the data from Data Buffer instance.
   . Backwarding
   . Display progress ( Data Buffer member funciton )
   . Validate if it is enabled.
- Add Classification Example with train member function

**Self evaluation:**
1. Build test:	 [X]Passed [ ]Failed [ ]Skipped
2. Run test:	 [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: jijoong.moon <jijoong.moon@samsung.com>