replace `pg_database` definition with LWRP
replace `pg_database_extensions` with LWRPs:

* add `postgresql_extension` LWRP
* add `postgresql_language` LWRP
* update `pg_database` recipe
* add spec coverage
* add ChefSpec matchers
replace `pg_user` definition with LWRP
strict about rspec deprecations
update `database` provider, add spec coverage
add `guard-rubocop`
cleanup Guardfile
remove default action
ensure action is a symbol
avoid unnecessary shell execution
rename `recipes/{pg_user -> setup_users}.rb`
rename recipes/{pg_database -> setup_databases}.rb
add `setup_extensions` recipe
add `setup_languages` recipe
update README documentation
add `contrib` recipe to extension setup
update dependencies, add `license_finder`
configure `license_finder`
update working Ruby version
update `rubocop` configuration