Issue #92 New refactoring - Remove useless null checks

RemoveUselessNullCheckRefactoring.java, RemoveUselessNullCheckSample.java: ADDED

AllRefactorings.java:
Added RemoveUselessNullCheckRefactoring.

ASTBuilder.java:
Added assign().

ASTHelper.java:
Reimplemented match(ASTMatcher, ASTNode, ASTNode) + removed alternative less efficient match() methods.
Fixed local test failure due to method visibility

OperatorEnum.java:
Changed compareTo(OperatorEnum, OperatorEnum) visibility to public.