VALARM custom new() methods

[niccokunzmann]

VALARM can be

• email
• display
• sound

according to RFC 5545.

Please create specific new_...() methods to verify all fields are set properly for these cases.

• create a nice doc string for them
• document parameters
• link to RFC
• give an example of how to use the class method adding this to an Event with Event.new()

[niccokunzmann]

Thanks @klouds27 for solving this! It looks nice!