add NAComparator for Rcomplex type

Hi All,
It seems that in the current version of Rcpp, member function `Vector::sort()` of `Vector` does not support complex vectors. This patch adds a comparator for the `Rcomplex` type, and it should work now. An example is given below:
```
> library(Rcpp)
> cppFunction('
+ 
+ RObject sort_complex(ComplexVector x)
+ {
+     ComplexVector xx = clone(x);
+     xx.sort();
+     return xx;
+ }
+ 
+ ')
> 
> r = i = c(-1, 0, 1, NA, NaN)
> comb = expand.grid(r = r, i = i)
> s = complex(real = comb$r, imaginary = comb$i)
> sort(s, na.last = TRUE)
 [1]  -1-  1i  -1+  0i  -1+  1i   0-  1i   0+  0i   0+  1i   1-  1i   1+  0i
 [9]   1+  1i       NA NaN-  1i       NA NaN+  0i       NA NaN+  1i       NA
[17]       NA       NA       NA       NA  -1+NaNi   0+NaNi   1+NaNi       NA
[25] NaN+NaNi
> sort_complex(s)
 [1]  -1-  1i  -1+  0i  -1+  1i   0-  1i   0+  0i   0+  1i   1-  1i   1+  0i
 [9]   1+  1i NaN+NaNi       NA   1+NaNi   0+NaNi  -1+NaNi       NA       NA
[17]       NA       NA       NA NaN+  1i       NA       NA NaN-  1i       NA
[25] NaN+  0i
```

There are a few inconsistencies between the two versions of sorting, possibly due to different treatment of ties (involving missing values and NaN) in R and `std::sort()`.