Custom GetConstructor method for cross platform support

[LeeCampbell]

Helps with #31.This means we are compatible with the following frameworks (according to the .NET Portability Analyzer)

• [ ] .NET v4.0 - Not a priority. Any OS post WinXP can run a runtime higher than 4.0
• [x] .NET Framework v4.5, v4.5.1, v4.5.2, v4.6, v4.6.1
• [x] .NET Core v5.0, .NET Core Cross Platform v5.0
• [x] .NET Native v1.0
• [x] ASP.NET 5 v1.0
• [x] Mono v3.3
• [x] Windows 8.1
• [x] Windows Phone v8.1