complex values

It should be straightforward to implement the complex case.  In response to a stackoverflow questin, Dirk gives this:

```
R> cppFunction("ComplexVector doubleMe(ComplexVector x) { return x+x; }")
R> doubleMe(1+1i)
[1] 2+2i
R> doubleMe(c(1+1i, 2+2i))
[1] 2+2i 4+4i
R> 
```
