Fixing bugs.

RemoveFieldsDefaultValuesRefactoring.java, RemoveFieldsDefaultValuesSample.java:
Do not remove non null initializers.

CommentsRefactoring.java, CommentsSample.java:
Ensured periods are added at the end of lines (not at the end of tags).
Do not add a period when first line of javadoc ends with period, exclamation mark, question mark or column.

ASTHelper.java:
Added isNullLiteral() + used it in RemoveUselessNullCheckRefactoring.