allow setting random_page_cost (and seq_page_cost)

as potentially recommended for solid state drives by http://www.cybertec.at/better-postgresql-performance-on-ssds/ and http://www.databasesoup.com/2012/05/random-page-cost-revisited.html
Merge pull request #109 from seamusabshere/patch-3

allow setting random_page_cost (and seq_page_cost)