Hstore support

Added Postgres hstore support.  Detects the pg gem in the gemfile on 'rails g impressionist' and asks the user if they want hstore support.  If so, it will add the hstore gem, run hstore:setup and create a migration to add a 'params' hstore column and index. It will then serialize all request params to the params column.  An Impression scope, Impression.with(key, value), enables searching for impressions by the params hash, by either key or key/value.