Easing installation process.

- Moved manifest to top level directory so users can just load the unpacked
  extension directly from whatever directory they git-clone into.
- INSTALL script has the /bin/sh shebang.
- INSTALL script replaces the "PATH_TO_EXTENSION" placeholder in the
  org.vim.stayfresh.json manifest file with the path to whatever directory
  the user git-cloned into.
- INSTALL script automatically copies the host/org.vim.stayfresh.json
  manifest file to the correct location on OSX.