Hit link attendee email address when showing events: Bug #740092

This moves some of the code out of util/util-uri.vala into a new
email utility file, since the code was not entirely related to URIs.
Improved URI parsing

Job Gruber updated the URI-parsing regex we were using in February
2014, so moved to it.  This new regex also allows us to catch mailto:
URIs.  The linkify code now allows transformation of the URI itself
when it's linked, so (for example) we could in the future ellipisize
long URIs and use a tooltip to give the user the full string.