Moved isMethod() from VectorOldToNewAPIRefactoring to ASTHelper + used it in several refactoring rules.

ASTHelper.java:
Moved isMethod() here from VectorOldToNewAPIRefactoring.

BigDecimalRefactorings.java, SimplifyExpressionRefactoring.java, StringBuilderRefactoring.java:
Fixed badly behaving refactorings that wrongly reported changes had been made.

CommentsRefactoring.java:
Added visit(CompilationUnit) + moved most code from getRefactorings() here to fix a NPE introduced with the AggregateASTVisitor.
Fixed badly behaving refactorings that wrongly reported changes had been made.

Refactorings.java, ASTCommentRewriter.java:
Consequence of the change to CommentsRefactoring.
Used Set instead of List for the instance member holding Javadocs to avoid duplicates because Javadocs are visited twice via CompilationUnit.getCommentList() and visit(Javadoc).
In ASTCommentRewriter, changed removals from List<ASTNode> to List<Comment>.

Pair.java:
Consequence of the change to Refactorings.
Added equals() and hashCode() to be able to use it within a HashSet.

ReduceVariableScopeRefactoring.java:
Removed unused import.