Update for dropbox api v2

[scps950707]

I fix three problems I met,

1. From dropbox-python-sdk:"File upload endpoints no longer accept file-like objects. Only byte strings are allowed", so I modify the argument of files_upload
2. From the discussion here, files_get_metadata API v2 is not allowed to check root folder's metadata, so I replace it with files_list_folder to modify listdir function
3. According to official doc, files_upload isn't used to upload single file larger than 150MB, so I use files_upload_session_start,files_upload_session_append and files_upload_session_finish to implement large file upload, I tested files range from 600~800MB and works well, resolves issue Can't upload large files (>200 MB) to Dropbox #301

[scps950707]

Sorry, I didn't find out these works done in #302