Implemented post increment and decrement operators.

[brokenprogrammer]

Added support for post increment and decrement operators. Resolves #4

>> let a = 4;
>> say a++;
5
>> let b = 10;
>> say b--;
9
>> class SomeClass { SomeClass(x) { this.x = x; } }
>> let q = SomeClass(100);
>> say q.x;
100
>> q.x = q.x--;
>> say q.x;
99