coding style update (#2752)

* coding style update

Current coding style varies(for example: the mixed use of single quote and double quote), and it will be confusing, especially for new users.
This PR will try to follow proposal of PEP8, make the documents more readable.

* minor fix

doc/python/python_intro.md

@@ -49,7 +49,7 @@ dtrain = xgb.DMatrix(csr)
 * Saving ```DMatrix``` into a XGBoost binary file will make loading faster:
 ```python
 dtrain = xgb.DMatrix('train.svm.txt')
-dtrain.save_binary("train.buffer")
+dtrain.save_binary('train.buffer')
 ```
 * Missing values can be replaced by a default value in the ```DMatrix``` constructor:
 ```python
@@ -106,7 +106,7 @@ bst.dump_model('dump.raw.txt','featmap.txt')
 A saved model can be loaded as follows:
 ```python
 bst = xgb.Booster({'nthread': 4})  # init model
-bst.load_model("model.bin") # load data
+bst.load_model('model.bin')  # load data
 ```
 
 Early Stopping