Context manager to temporarily change to a new directory.

    http://lucentbeing.com/blog/context-managers-and-the-with-statement-in-python/

def chdir(new_dir):
    """Context manager to temporarily change to a new directory.

    http://lucentbeing.com/blog/context-managers-and-the-with-statement-in-python/
    """
    # On busy filesystems can have issues accessing main directory. Allow retries
    num_tries = 0
    max_tries = 5
    cur_dir = None
    while cur_dir is None:
        try:
            cur_dir = os.getcwd()
        except OSError:
            if num_tries > max_tries:
                raise
            num_tries += 1
            time.sleep(2)
    safe_makedir(new_dir)
    os.chdir(new_dir)
    try:
        yield
    finally:
        os.chdir(cur_dir)