Support for native and mixed mode assemblies

#### Mixed assemblies
These must be loaded from disk as the CLR does not support loading unmanaged code from memory.

http://stackoverflow.com/questions/2945080/

#### Unmanaged assemblies
These must also be loaded from disk. And they cannot be referenced the normal way. So, they will work if the user manually embeds the reference into either a `costura32` or `costura64` folder in the assembly. The loader code will load from the correct folder based on the bittyness of the process at runtime.

#### Config options
There is now an `Unmanaged32Assemblies` and an `Unmanaged64Assemblies` option. They work like the `IncludeAssemblies` and are required for unmanaged and mixed mode assemblies. They tell Costura which assemblies are unmanaged and what bittyness to load them into.