Added Dropbox integration

I missed a function from Windows' Jing that uploads your screenshot to an online service and places the URL for the image in your clipboard. I find that feature critical for fluid communication which is why I diceded to add a similar feature to Deepin Scrot with the help of Dropbox. Take a look at the README file and feel free to pull whatever code you like from my hack into the main project. I think a lot of users could benefit from this (and not only in Linux Deepin but in most other distros).

Thanks so much for this tool. It's really good and has helped me a lot!

Santiago