A few fixes for the Makefile:
 * Lose USE_PGXS, per modern custom
 * bail out cleanly on versions of PostgreSQL < 9.3

Some logic adapted from pg_repack's Makefile.
Merge Makefile contribution from Josh (schmiddy).

I did minor edits to the Makefile, and some new additions to the README.

Merge commit 'c393236a275109b4d2a2c2902898cbf4f18f62a1'