Allow JUnit ClassRule to be public

[avano]

The JUnit ClassRule should be treated the same way as JUnit Rule, as must should be declared public.

Currently only @rule is ignored from the check and latest version complains about:
Variable 'myClassRule' must be private and have accessor methods. [VisibilityModifier]

Class:

/tmp/checkstyle :: cat Example.java
public class Class {
@org.junit.Rule
public Rule rule = new Rule();

@org.junit.ClassRule
public static Rule classRule = new ClassRule();
}

XML:

/tmp/checkstyle :: cat checkstyle.xml
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
"-//Puppy Crawl//DTD Check Configuration 1.3//EN"
"http://www.puppycrawl.com/dtds/configuration_1_3.dtd">

<module name="Checker">
<module name="TreeWalker">
    <module name="VisibilityModifier"/>
</module>

Outcome:

/tmp/checkstyle :: java -jar checkstyle-6.16-all.jar -c checkstyle.xml Example.java
Starting audit...
[ERROR] /tmp/checkstyle/Example.java:6:28: Variable 'classRule' must be private and have accessor methods. [VisibilityModifier]
Audit done.
Checkstyle ends with 1 errors.

JUnit rule is in the 'whitelist' in VisibilityModifierCheck class:

/** Default ignore annotations canonical names. */
private static final List<String> DEFAULT_IGNORE_ANNOTATIONS = ImmutableList.of(
    "org.junit.Rule",
    "com.google.common.annotations.VisibleForTesting"
);

but the ClassRule is not, even if it's the same principle.

[romani]

fix is merged,
thank a lot for full update (code + documentation).