Timezone tests changed to take into account daylight saving time. 2 tests were failing, because offset between UTC and London timezone is not constant due to DST(0 with DST off, 1 with DST on).
One from strings tests was failing due to inconsistent declaring DateTime COOKIE constant across multiple PHP versions. Check here http://3v4l.org/Pu6TS#v520
Self keyword changed to static in all method calls, but all constants references were left with self keyword. It is due to the fact that there are constants, but also because static keyword is not allowed in compile time constants. Therefore static keyword can't be used to declare another constant or method argument.
Fixing condition in testToCOOKIEString() method.
Merge pull request #121 from doubleking/master

Method calls with self keyword changed to static. Also few tests fixes.
Released 1.9.0
Add useful decade and century add, and diff functions
Also replace integers with constants