feature: default culture option in AppSettings

[gribkovav]

DefaultCulture class lacked the ability to set its value - it was hardwritten value of 1033 (en-US locale), though in many cases for other (not from Knoema 😄 ) developers keys (text) for localization may be originally presented in other language.
Now DefaultCulture has its default value set to en-US but it can be configured via <appSettings> in the following way inside of web.config:

<appSettings>
   ...
   <add key="localizerDefaultCulture" value="ru-RU"/>
</appSettings>

Changes description:

• Knoema.Localization.Core.csproj :added reference to System.Configuration
• DefaultCulture.cs: changed the implementation of DefaultCulture.Value (no need for multithreaded sync - its static context and immutability during app lifecycle).

[MikhailPikhota]

Please, made edit a DefaultCulture to non en-US or DELETE a DefaultCulture.
If DefaultCulture is set, please do not create a lines in DB.

[MikhailPikhota]

In this file (Localization.Core.dll ver. 1.94) I made a changes DefaultCulture to ru-RU.

Knoema.Localization.Core.zip