Find the directory PREF correctly when installed in user directory

When installed with `pip3 install bcloud --user`, PREF should be $HOME/.local/share
use os.path.join to join paths
Merge pull request #78 from wzhd/master

安装在用户目录下时, 找到PREF目录的正确位置
Fixed: failed to load bcloud/bcloud.png; https://github.com/LiuLang/bcloud/pull/78