Add sequential stubbing

[searls]

Often you want to stub different values for the first, second, third, etc., invocation of a test double. Support this with variable arguments on thenReturn

when(testDouble(8)).thenReturn(1,2,3)

testDouble(8) //1
testDouble() // undefined
testDouble(8) //2
testDouble(8) //3
testDouble(8) //3

Subsequent invocations beyond those specified will continue to return the last value.