Return class instance on setRDates and setExDates (simshaun#135)

CS update

Full support for immutables (simshaun#123)

* Run ArrayTransformerConstraintTest with using either \DateTime or \DateTimeImmutable

* Full support for immutables

- Method signatures changed from \DateTime to \DateTimeInterface

- As a consequence, drops support for PHP 5.3 and PHP 5.4

adding test for TZ_FIXED behavior of getString (simshaun#116)

Fix for for bysetpos transformation (simshaun#114)

* failing tests for bysetpos transformation
* adding missing throws annotation
* removing unused var
* fixing empty index error
* checking for empty daySet (throws E_WARNING < 5.4)

Fixes simshaun#121

Infinite rule check (simshaun#113)

* adding missing @throws annotation

* returning object from array loader (and string loader) to match interface

* updating docblock to match behavior

* removing unused temp var

* strlen will always return an int so === is safe

* adding method to check for infinite rule recurrence without having to build 732 and guess

* reverting back to long array syntax so the travis 5.3 build is happy

Added index property to Recurrence (simshaun#110)

* Added an index property to Recurrence

* Added test for index

* Removed typehint

* Added intial index property

Fixes simshaun#106