Fixed a bug in the preferences page where the wrong prerefence name and label was used.

PreferenceConstants.java:
Added a Preference class that stores a preference name and its description altogether.

*.java:
Added helper methods.
Issue #58 Do not remove parentheses in integer addition

SimplifyExpressionRefactoring.java:
In getExpressionWithoutParentheses():
- used OperatorEnum.compareTo()
- removed a lot of code now redundant
- bettered isHardToRead()
Removed isReadable()
Moved isAssociativeOperator() to OperatorEnum.isAssociative().
Moved isBooleanOperator() to OperatorEnum.isBoolean().
Added a static import for ASTHelper.

OperatorEnum.java, OperatorEnumTest.java: ADDED

SimplifyExpressionSample.java:
Added one test case.