add code and project files to build DLR
add several fixes for .NET 3.5

Fix unsupported classes, interfaces, methods and concepts.
add configs for project and use custom DLR if specified
update .travis.yml: support .NET 3.5
small fixes to 3.5 compatibility
one more 3.5 fix: change String.Concat() overload

System.String.Concat(IEnumerable<string>) was introduced in .NET 4.0,
but all versions support String.Concat(string[]).