fix package_manager.py non-ASCII problem

While install new package on Win7 with login account named in CJK, I can't install package. The error in Sublime console was:



    Traceback (most recent call last):
      File ".\threading.py", line 532, in __bootstrap_inner
      File ".\package_control\package_installer.py", line 244, in run
      File ".\package_control\package_manager.py", line 476, in install_package
      File ".\ntpath.py", line 108, in join
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xcd in position 9: ordinal not in range(128)

    ignored packages updated to: [Vintage]

So I figuered out that the file path will throw an unicode exception since the `package_name` variable was unicode fron JSON all the way down . This patch fixes the problem and the path will be str().