Expand tilde-paths for calling hoogle locally

The current document suggests adding path elements like
"~/.cabal/bin" (a.k.a. "tilde-paths") to the PATH environment variable
and the `exec-path' variable.  I had some trouble with this when using
zsh as my default shell -- in particular, efforts to call hoogle locally
would error out with the message: "zsh:1: command not found: hoogle".
There were no such problems when I used bash as my default shell.

A reasonably robust solution to this problem is to wrap tilde-path
elements like this in a call to `expand-file-name', which will expand
them to absolute paths - a situation which I'd expect every shell to
handle properly.  (I've tested with both bash and zsh and they, at
least, have no problem with this.)
Merge pull request #17 from dpkatz/master

Make paths for calling cabal executables more robust