Don't invoke el-get if el-get invoked us

This commit implements solution number 4 on issue #16. Emacs's backtrace-inspection features aren't the most pleasant to work with, but this code worked on both my normal .emacs.d setup (where el-get invokes elhome) and an empty one that I set up according to the docs (install elhome using elhome-install, and then put `(load "el-get/elhome/elhome.el") (elhome-init)` in `init.el`).

How do you feel about this?