#38 - Lower required version of .NET

As discussed in the comments of #38 this commit pulls femah's .NET
framework dependency down to .NET 3.5.
To note:
- We no longer have IsNullOrWhitespace() available to us from .NET 4.0
so I shifted two usages to IsNullOrEmpty()
- Implemented my own Enum.TryParse equivalent, once we dealt with
case-sensitivity that worked fine (unit tests picked it up)
- Updated all existing Nuget references to use the .NET 3.5 assemblies
as availble