Change make shell to make repl

The former starts an Erlang shell and the latter starts an lfe REPL
like described in the text.
README file is now formatted as Markdown

Also added a few missing files that show up when running `git status`.
Update quick start examples to match code

The generated unit test code and messenger-back code changed slightly
from when the quick start guide was written. This change makes them
match again.
Removed note about updating ERL_LIBS

Since resources/make/common.mk runs `lfetool info erllibs` and
sets the ERL_LIBS variable itself, it's no longer necessary (or
possible) to do this.
Fix typo: ny -> by