Add support for builder.AddAppSettings

[tsimbalar]

First part of #5 : AppSettings

Support for loading key value pairs from an arbitrary config file.

Notes :

• The AppSettingsSettings.cs file has been copied and adapted from Serilog.Settings.AppSettings for now.
• All the AppSettings related part is surrounded with #if APPSETTINGS to make it available only on full framework for now. I don't know if that could be problematic and if there is a better way
• This is a "repackaging" of a bigger bunch of commits, and to make things easier, I have put all the creation of Test support stuff (TestDummies, Tests.Support etc) in the first commit.

[nblumhardt]

👍 thanks for the split PRs, a bit easier to digest :-)