add execv

[ If I tag @armstrtw here, will he get notified? ]

This patch solves Whit's problem as reported here: http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2014-June/007766.html

Example:

```R
> redis$exec("SMEMBERS hats")
list()
> redis$exec("SADD hats 'big hat'") ## quoting doesn't work
[1] 2
> redis$exec("SMEMBERS hats")
[[1]]
[1] "hat'"

[[2]]
[1] "'big"

> redis$execv(c("SADD", "hats", "little hat")) ## vectorizing does work
[1] 1
> redis$exec("SMEMBERS hats")
[[1]]
[1] "little hat"

[[2]]
[1] "hat'"

[[3]]
[1] "'big"
```