Add Support for Storing Values as Strings in the Database

This will add the ability to specify (at runtime) whether you would like to save MACs as strings (varchar) or integers (bigint) via an "integer" keyword (defaults to True, i.e. existing behavior). I've also reworked how to specify a default dialect (via a settings variable and a utility function that handles importing it), and added a function to format any given EUI instance via a specified mac_eui48 subclass. I know that my change to how a default dialect is set is backwards-incompatible, but I think it's more elegant solution than using a class method.