A few pypy improvements

I have not yet used pypy, but it would be good to have them in the standard buildout.cfg and have bin/install-links working.

Note that I have one git clone for each python version and have this in a local.cfg, where instead of pypy3 I also have clones with python2.7, etc:

```
[buildout]
extends =
    buildout.cfg
parts =
    ${buildout:base-parts}
    ${buildout:readline-parts}
    ${buildout:libjpeg-parts}
    ${buildout:pypy3-parts}
    ${buildout:links-parts}

[install-links]
prefix = /Users/mauritsvanrees/py
```

With this setup and the code from the pull request, I get this output when running bin/install-links in the 2.7 buildout:

```
$ bin/install-links 
replace /Users/mauritsvanrees/py/bin/virtualenv-2.7? y
/Users/mauritsvanrees/py/bin/virtualenv-2.7 -> /Users/mauritsvanrees/buildout/python2.7/bin/virtualenv-2.7
ls: /Users/mauritsvanrees/buildout/python2.7/pypy*/bin/pypy: No such file or directory
replace /Users/mauritsvanrees/py/bin/easy_install-2.7? y
/Users/mauritsvanrees/py/bin/easy_install-2.7 -> /Users/mauritsvanrees/buildout/python2.7/python-2.7/bin/easy_install-2.7
replace /Users/mauritsvanrees/py/bin/pip-2.7? y
/Users/mauritsvanrees/py/bin/pip-2.7 -> /Users/mauritsvanrees/buildout/python2.7/python-2.7/bin/pip-2.7
replace /Users/mauritsvanrees/py/bin/python2.7? y
/Users/mauritsvanrees/py/bin/python2.7 -> /Users/mauritsvanrees/buildout/python2.7/python-2.7/bin/python2.7
```

In the pypy3 buildout it becomes:

```
$ bin/install-links 
replace /Users/mauritsvanrees/py/bin/virtualenv-pypy3? y
/Users/mauritsvanrees/py/bin/virtualenv-pypy3 -> /Users/mauritsvanrees/buildout/pypy3/bin/virtualenv-pypy3
ls: /Users/mauritsvanrees/buildout/pypy3/python-[23].*/bin/*[23].*: No such file or directory
replace /Users/mauritsvanrees/py/bin/pypy? y
/Users/mauritsvanrees/py/bin/pypy -> /Users/mauritsvanrees/buildout/pypy3/pypy3/bin/pypy
```

Note that pypy3/bin/ also has files `easy_install-3.2`, `pip-3.2` and `python3.2`. I have setup the `src/install-links.in` file to not copy those, as they would interfere with the files from 'normal' python3.2.