Add removeColumn method in DDF
Update DDF.java

Add judgement of input parameter and correct a mistake
add removeColumn and removeColumns methods
remove DDF removeColumn method
Merge branch 'master-qxw' into mat
Update DDF.java
Update ViewHandlerTest.java
Merge branch 'master' into mater
Update removeColumn method and test
Merge pull request #50 from qinxinwei/mater

Add removeColumn method in DDF