A simple QuerySelectorAll command

In response to https://bugzilla.mozilla.org/show_bug.cgi?id=770156 and based on https://gist.github.com/3000231 here is a simple queryselectorall command that could be interesting to you.

It doesn't do much for now apart from listing the matched elements and giving the number of elements matched.

Here is a screenshot:
![qsa command screenshot](https://raw.github.com/captainbrosset/gcli/images/qsa.png)

In the gcli version embedded in the Fx devtools, I know there's an `inspect` command too, but I think it could benefit from listing all matches rather than forcing the user to write a selector that matches only one element.

Anyway, here's the pull request, up to you.