Add libpe as a submodule now that it's an independent repository.

Now people that want to clone `pev` using Git >= 1.6.5, should proceed as follows:

    git clone --recursive https://github.com/merces/pev.git

For older versions, proceed as follows:

    git clone https://github.com/merces/pev.git
    git submodule init
    git submodule update

Thanks to @felipensp for pointing out the `--recursive` option!