Simplified argument captor API (originally #242)

[nhaarman]

This will simplify ArgumentCaptor code

        //old (still working)
        argumentCaptor<String>().apply {
            verify(mailRepository).sendEmail(capture())
            firstValue shouldEqual "A"
        }

        nullableArgumentCaptor<String>().apply {
            firstValue shouldEqual null
        }

        // new simplified overload usage
       argumentCaptor<String> {
            verify(mailRepository).sendEmail(capture())
            firstValue shouldEqual "A"
        }

        nullableArgumentCaptor<String> {
            firstValue shouldEqual null
        }

This is a fixed version of #242, rebased onto 2.x.