Move prelude-modules.el under personal directory

Prelude modules are required by the definition file `prelude-modules.el`.
This file should be part of personal configuration.

Therefore:

- implemented loading of `personal/prelude-modules.el` in favour of
  loading `prelude-modules.el`;
- added a check for presence of both old and new files;
- adjusted documentation;
- adjusted the installer.

Closes #1206.

.gitignore

@@ -15,7 +15,6 @@ custom.el
 places
 .smex-items
 savefile/
-/prelude-modules.el
 projectile-bookmarks.eld
 session*
 .cask