Added null parameter guard.
Uninstalled unused nuget package - moq.
Supported the CustomizeAttribute feature.
Added the constructor having fixture type.
Added the XML document.
Renamed fixture to builer.
Implmented auto data testing

using specimen builder type.
Resolved the code analysis errors.
Renamed fixture to builder.
Refactored SpecAttribute to improve performance,

Xunit framework calls many times a constructor of the attributes drived
from FactAttribute. So this commit simplied the constructor to improve
performance.
Refactored SpecAttribute.
Refactored SpecAttribute.
Tested fixture initialized only once.
Renamed the test types.
Deleted the protected constructor,

which will cause to initialize an instance of IFixture many times.
Implemented the protected Builder property.
Added the XML document.