Adding support for deep queries.

This version changes `IElasticMapping` to take `MemberExpression` instead of `MemberInfo`.

There are two issues here:

- There are 2 failing facets tests (which probably means I broke facets). Damien will need to look at that to understand how much of an idiot I am.
- There is only the single test for deep property accessors. That needs to be moved to the right place and/or expanded to support all expression types (not just equals), since in theory deep property accessors are valid everywhere. :)