Modify Homebrew path config setup (OS X)

The old Bash config was setting up duplicate entries for
`/usr/local/bin`, in combination with the preset `/etc/paths` file. This
is fixed by using [this Super User solution][1].

This commit also adds the `list-paths` function to the Bash config.

[1]: http://superuser.com/a/580611/86416
Fix Sublime Text PATH config (OS X)

The old config was causing strange behavior with PATH, so this new
config avoids modifying PATH altogether.