Register link on event information pages needs to be more prominent by default

Adding markup to style this link as a button so it will be more visible on the page.
Also included crm-register-button class which allows themers / designers to over-ride the default button look / color and apply their own css style to it w/o modifying the template.

<a href="{$registerURL}" title="{$registerText}" class="button crm-register-button"><span>{$registerText}</span></a>