README.md: Document _env: syntax defaults

Catch the docs up with 7762af1 (Added support for environment variable
default value, 2013-12-09).
run.sh: Remove --debug argument

This doesn't do anything constructive in Gunicorn v18.0, and having it
around just interferes with --preload [1].

[1]: https://github.com/benoitc/gunicorn/pull/701
run.sh: Add '$GUNICORN_OPTS "$@"' to gunicorn invocation

This allows users to add custom arguments via the command line ($@) or
environment ($GUNICORN_OPTS), which is useful if you want to --preload
(or tweak some other setting) without editing your config file.
search: Use SQLAlchemy as a search-index backend

This avoids storing search-index state in the local process (as the
previous Python-dict-based implementation did).  Multiple registry
processes (e.g. gunicorn workers) can now use the same search-index
database and stay in sync with each other.