v2.7.0 - File tree structure and optimizations

Added --tree option to save files in a tree structure: YYYY/MM/DD.png instead of YYYY-MM-DD.png.
Proxy server is no longer pinged when downloading <10 images. This can be changed with --always-ping.
Reimplemented human_bytes, humantime, and fs_extra crates, to remove single-use dependencies.
Renamed --notify-fail option to --notify-on-fail.

Minor changes:

• Fixed conflicts with --source and --cache arguments
• Refactored error handling to use an enum
• Converted primary download function to a method, and reorganized configuration types
• Added basic tests for reimplemented functions

Full Changelog: v2.6.0...v2.7.0