Make `package-user-dir` relative to Prelude

This makes the entire Prelude configuration portable, meaning it's easier to try
out in different Emacs enviroments.
auto-indent code pasted in clojure and scala buffers
added a basic list of useful keybindings available in Prelude
fixed #235 - shell variables have to be exported to be accessible in subshells
fixed #215 - made Prelude's auto-save optional (and improved it a bit in the process of doing so)
fixed #228 - restored prelude-personal-snippets-directory
enabled flyspell only is aspell is available
added variables that control the enabling/disabling of guru and whitespace-mode
adjusted the README to reflect recent configuration changes
make flyspell easier to disable
Update prelude/prelude-editor.el

fixed wrong type arument: symbolp, convenience
fixed a bug with the prelude-flyspell check
whitespace cleanup
don't hide git clone errors
started to replace cl code with dash.el
migrated a bit more code to dash.el
moved back the prelude modules into the main Prelude project
fixed #241 - fixed a typo in a function name
fixed #242 - restored prelude-add-subfolders-to-load-path
fixed load order issue