Minor tweaks and changes

I tidied up a few bits of the script.

* The location of OmniFocus is configured in line 3, to make life easier for users who don’t store OF in `/Applications`. (For example, `~/Applications` is a popular alternative.)
* Since `echo` ignores newlines, I changed this to `printf`, which does it better.
* I added an `uninstall.sh` script, which essentially does exactly what’s described in the README, but using the `$APP` variable and making things slightly easier.
* I edited the script so that it can be run from any working directory, not just `omnifocus-modernize-theme`.
* Changed the help text in the README and `install.sh` to better reflect the buttons presented to the user when they install `omnifocus2.ofocus-theme`.