Provide customization of the Id property convention

[bytenik]

I propose a lambda function passed into a configuration option, that given an object o will return the id.

[Mpdreamz]

Right now you can specify an alternative id by marking the type with a

[ElasticType(IdProperty="AlternateId")]

attribute.

I do like having a non attribute way to do it as well though since you don't always own your domain classes.