Fix typo
Add docstring to login
Merge branch 'master' into develop
Refactor to use print function
Add docstring to core
Merge branch 'print_function' into develop
Refactor to use print function
Join two path name
Use custom logger instead of logging

Now log could work. I will add verbose mode, debug output in console.
Fix logger error
Addd logging to login and change the logic
Rename log type
Fix issue of path expand and print function
Fix issue of secret link

If user copy the link from browser and the link is already convert to
the url containing 'init', it will exit. Now fix it.
Fix raise IndexError

If link has been deleted(by user or baidu), raise DownloadError rather
than a IndexError.
Merge branch 'develop'