Let-bind user-emacs-directory to cwd

Useful for testing locally, since now you can simply blow away the
`.stackmode' directory instead of searching for it in `.emacs.d'.

At any rate, it keeps everything in the project tidy.
Let-bind stack-cache-directory instead

There are several variables and functions that depend on it; simply
binding the filter-cache file isn't enough.
Implement and use `stack-cache-get' and `-set'