Member class should include IsReadOnly info

[BrannonKing]

Sometimes the properties returned from FastMember are read-only. This is more common now with the newer C# features for set-once properties. It would be nice to avoid the exception that occurs when trying to write to them.

I also like the other feature request to expose the reflection "info" class. However, I would prefer to not have to convert that to a PropertyInfo, check for the existence of a setter, etc.

Also, it would be nice if MemberSet had a string indexer for fast lookup of members by name.