Support multiple mapping resolvers per session

[TrevorPilley]

This will allow a mixture of convention and attribute based mapping plus any custom conventions defined by an end user. The resolution will work similar to how the routing works in ASP.NET WebApi e.g.

config.UseAttributeMapping();
config.UseConventionMapping(ConventionOptions.Default);

In this case, it would look for an attribute first, and then if one is not found, fall back to the conventions to resolve the mapping.