BUG Fixing tabindex added to CreditCardField when tabindex is NULL

The tabindex increment *should* only be done if there is a tabindex
that has been set on a CreditCardField already, otherwise it breaks
the tab ordering.
FIX Auto-escape titles in TreeDropdownField

Related to SS-2013-009. While the default "TreeTitle" was escaped
within the SiteTree->TreeTitle() getter, other properties like SiteTree->Title
weren't escaped. The new logic uses the underlying casting helpers
on the processed objects.
API Escape form validation messages (SS-2013-008)
Added 3.0.7 changelog