A better way to locate project creator

The use of `os.getcwd()+'/tools/project_creator/'` to locate project creator is problematic. This assumes that the user is executing the script from the cocos2d-x root directory:

    $ pwd
    /home/timothy/repo/cocos2d-x
    $ ./create-multi-platform-projects.py  # Okay
    $ cd ..
    $ cocos2d-x/create-multi-platform-projects.py  # Failed

The last try will produce '': No such file or directory".

I changed the way to locate cocos2d-x root directory by using the path of the script itself.

p.s. Also, `os.path.join()` is safer than connect strings manually.