EMail Fix

Outgoing e-mails are now UTF-8 by default.
Event PreRegisterHook

Implemented a protected PreRegisterHook method, so that derived classes can perform processing/validation immediately before a user is actually registered, taking advantage of the Event class' built-in locking facilities et cetera.

Previously this method existed, but was not actually called, and had no documentation.
Event Registration Details Formatting

Added methods to the Event class to format registration details for output, matching their database keys to descriptive labels, and optionally filtering the values to transform them (default transform reformats floating-point values as currency).
E-Xact

Added classes under the EXact namespace for handling a few E-Xact use cases:

- The HostedCheckout class encapsulates the information related to a hosted checkout, and verifies transactions associated with that hosted checkout by attempting to rederive the associated hash
- The Form class is a base class for E-Xact forms, providing shared and common functionality
- The SingleItemForm class handles the usual use case -- the user is purchasing/paying for one logical item