Run tests from assembly

Example:

```
var runner = Require<XunitRunner>();

var result =
runner.Execute(@"D:\projects\Tanka.Markdown\src\MarkdownTests\bin\Debug\Tanka.MarkdownTests.dll");

Console.WriteLine("Executed tests {0} - failed - {1} - total time {0}
seconds",
result.TestsCount,
result.TestsFailed,
result.TotalTimeInSeconds);
```