Added support for short variable name exceptions

Sometimes we need exclude variables like 'id' from short name checking.

New property for rule was introduced - 'exceptions'. It is comma-separated list of variables names to be excluded from short name checking.  

Example usage:
 ```xml
    <rule ref="rulesets/naming.xml/ShortVariable" >
            <properties>
                <property name="exceptions" value="id,em" />
            </properties>
    </rule>
```