Simple emacs integration

Hi

I saw the project on reddit and it seems pretty cool. I decided to write a small emacs integration to be able to use it in my normal work flow. I quickly realized I didn't get line numbers since stdin didn't look like a tty to ag. I couldn't find a way to circumvent this with the existing functionality (except using --ackmate and since I wanted to use the built-in grep support that wasn't a good option). Hence the first commit in the pull request. I've always felt that the emacs integration should sit next to the actual project so I figured I'd make it part of the pull request as well.

Nice work. Definitely faster than grepping.

Cheers

Anders