4.0.0-alpha1

• sabre/vobject now requires PHP 5.5.
• #244: PHP7 support.
• Lots of speedups and reduced memory usage!
• #160: Support for xCal a.k.a. RFC6321! (@Hywan)
• #192: Support for xCard a.k.a. RFC6351! (@Hywan)
• #139: We now accept DateTimeInterface wherever it accepted DateTime
  before in arguments. This means that either DateTime or
  DateTimeImmutable may be used everywhere.
• #242: Full support for the VAVAILABILITY component, and calculating
  VFREEBUSY based on VAVAILABILITY data.
• #186: Fixing conversion of UTC-OFFSET properties when going back and
  forward between jCal and iCalendar.
• Properties, Components and Parameters now implement PHP's JsonSerializable
  interface.
• #139: We now always return DateTimeImmutable from any method. This could
  potentially have big implications if you manipulate Date objects anywhere.
• #161: Simplified ElementList by extending ArrayIterator.
• Removed RecurrenceIterator (use Recur\EventIterator instead).
• Now using php-cs-fixer to automatically enforce and correct CS.
• #233: The +00:00 timezone is now recognized as UTC. (@c960657)
• #237: Added a destroy() method to all documents. This method breaks any
  circular references, allowing PHP to free up memory.
• #197: Made accessing properties and objects by their name a lot faster. This
  especially helps objects that have a lot of sub-components or properties,
  such as large iCalendar objects.
• #197: The $children property on components has been changed from public
  to protected. Use the children() method instead to get a flat list of
  objects.
• #244: The Float and Integer classes have been renamed to FloatValue
  and IntegerValue to allow PHP 7 compatibility.