Split DotKEGG into Domain and Operations Assemblies

[Rabadash8820]

DotKEGG needs to target .NET 3.5 so it can be used in Cell. However, most people using DotKEGG will probably be using .NET 4 or later, with access to the TPL and async/await programming, and will want those more modern features. Therefore, we should split DotKEGG into two assemblies:

1. DotKEGG.Domain: contains the types that are basically just POCOs, including KeggId and its subclasses, KeggDb and its subclasses, and all the future database entry classes. This assembly should target .NET Framework 3.5.
2. DotKEGG.Operations: contains the KEGG operation classes, including KeggInfo, KeggList, KeggGet, KeggLink, etc. There will be two versions of this assembly, one targeting .NET Framework 3.5, and one targeting .NET Framework 4 and later. Only the latter will support async methods.

Of course, both assemblies should be provided in the NuGet package. NuGet handles adding references to the proper assembly versions.