[postgres] remove (outer) sub-class declarations with nested modules/classes

... and require files **after** the enclosing class is actually defined

while this "dries" up the code nicely (there are no multiple `PostgreSQLAdapter < AbstractAdapter` declares) it also allows for possible code sharing between Rails' and AR-JDBC's PostgreSQL adapter (at AR-JDBC's own risk currently - well at least until Rails' travis-ci is green with JRuby)