added exception in case of missing initialization call
removed BrowserCoreTest as BrowserCore is no more
made IsBrowserInitialized a Dependency Property
added check because Application.Current is not set when running unittests
ported unit tests to use xunit
+ nuget.org as package source for xunit @Appveyor
Merge pull request #1 from jornh/patch-1

Add nuget.org as package source for xunit @Appveyor
Merge pull request #366 from JanEggers/Unittests

Unittests converted to xUnit