kijiproject.github.com: Update REST user guide.

Update all references to root KijiREST directory to be ${REST} (to
remove ambiguity between those using a BentoBox and those pulling the
REST tarball off of the website.

Replace start_rk and end_rk with start_eid and end_eid.

Add tips about what to do if your REST instance fails to start.
commit: https://github.com/kijiproject/kijiproject.github.com/commit/9488e70b941150c11cebf71bef5e6967b5b53d70