Adds .NET Framework support

[georgechond94]

This PR allows the wrapper to be used in projects targeting the full .NET Framework.

• When using CallingConvention = CallingConvention.Cdecl, PInvokeStackImbalance MDA (Managed Debugging Assistants) throws an exception on every P/Invoke call with the message:

  Managed Debugging Assistant 'PInvokeStackImbalance' 
  A call to PInvoke function 'WkHtmlToPdfDotNet!WkHtmlToPdfDotNet.WkHtmlModule::wkhtmltopdf_init' has unbalanced the stack. 
  This is likely because the managed PInvoke signature does not match the unmanaged target signature. 
  Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

  This is not the case with .NET Core as the MDAs don't exist.

I replaced every CallingConvention with CallingConvention.StdCall (default), which was working fine on my environment (Windows, .NET Framework & Core) but it would be great if you could verify that it's working on every other environment as excepted as well. :smile:

• An extra src/WkHtmlToPdf-DotNet/build/Haukcode.WkHtmlToPdfDotNet.targets added which should copy the /runtimes/win-x DLLs to the output folder of the referencing application on build, as I couldn't make it to work the same way it does on .NET Core. Current integration with .NET Core isn't affected by that.

Closes: #33, #28 (?)

[HakanL]

Very nice, yeah I'm not sure why it was using Cdecl, it was like that in the original repo and I just assumed it was correct. I've tested your PR in Docker and it works fine, and the changes make sense. Thank you very much for your contribution!