fixing symlink issue

When calling oalab, I got this error:
```
Traceback (most recent call last):
  File "/home/cokelaer/Work/virtualenv/bin/oalab", line 9, in <module>
    load_entry_point('OpenAlea.OALab==0.0.2', 'gui_scripts', 'oalab')()
  File "/home/cokelaer/Work/github/openalea/oalab/src/openalea/oalab/main.py", line 36, in main
    create_project_shortcut()
  File "/home/cokelaer/Work/github/openalea/oalab/src/openalea/oalab/project/symlink.py", line 55, in create_project_shortcut
    symlink(oalab_dir,project_link_name)
  File "/home/cokelaer/Work/github/openalea/oalab/src/openalea/oalab/project/symlink.py", line 36, in symlink
    os_symlink(source, link_name)
OSError: [Errno 17] File exists
```

The error is due to the fact that the symlink being created already exists. Here is a possible fix.