Defer connection until first use, instead of at configuration-time.

This is necessary for use with Thin, as it doesn't start the EventMachine reactor until the first request.
Note that, by default, the Railtie will still force the connection at initialization-time, but this can be overriden.

lib/mongoid/config.rb

@@ -103,7 +103,10 @@ def master=(db)
     #
     # The master +Mongo::DB+
     def master
-      raise Errors::InvalidDatabase.new(nil) unless @master
+      unless @master
+        _master(@settings)
+        raise Errors::InvalidDatabase.new(nil) unless @master
+      end
       if @reconnect
         @reconnect = false
         reconnect!
@@ -142,6 +145,7 @@ def slaves=(dbs)
     #
     # The slave +Mongo::DBs+
     def slaves
+      _slaves(@settings)  unless @slaves
       @slaves
     end
 
@@ -194,8 +198,7 @@ def from_hash(settings)
       settings.except("database", "slaves").each_pair do |name, value|
         send("#{name}=", value) if respond_to?("#{name}=")
       end
-      _master(settings)
-      _slaves(settings)
+      @settings = settings.dup
     end
 
     # Adds a new I18n locale file to the load path