Fixed: constant name was misleading.

Previously CPTimerMinTimeInterval was used as a constant for the *default* time interval used when a timer was initialized with an interval <= 0. It isn't the minimum, it is possible to create a timer with an interval less than that constant's value.

The name has been changed to CPTimerDefaultTimeInterval to make it clearer what it's purpose is. It has also been changed to a #define, that will result in smaller generated code.