Improve the grammar of the Dev donation setting.

- move source code to `src`
- categorize files and move to group folder
- change upper case class files to lower case
- change C++ header to `*.hpp`

based on the reddit poll we get 45.99% support to increase the developer donation
https://www.reddit.com/r/MoneroMining/comments/6eu00j/question_xmrstak_default_donation_increase_to_2/