Add ability to configure id properties in code

[gmarz]

Previously, the only way to specify an id property was to use the ElasticTypeAttribute:

[ElasticType(IdProperty="AlternateId"]

This PR introduces an alternative way to configure id properties, via code:

var settings = new ConnectionSettings()
    .MapIdPropertyFor<Foo>(o => o.AlternateId);

Note: Mapping two different id properties on the same type is an error:

var settings = new ConnectionSettings()
    .MapIdPropertyFor<Foo>(o => o.AlternateId)
    .MapIdPropertyFor<Foo>(o => o.AnotherId);

and will cause an ArgumentException to be thrown.

Question: How should we treat the case where a type has an Id property set via ElasticTypeAttribute and an id property is mapped in code? Currently, the code mapping will take precedence, but I wonder if this should be an error instead?

Closes #407

[Mpdreamz]

LGTM!

I do not think it should be an error and the current precedence makes sense. In the esoteric case where a type is marked with an ElasticTypeAttribtute and through a ConnectionSettings mapping. The ConnectionSettings mapping is the most "local" and the only way to override an already marked type (possible in a compiled assembly).