Release version 2.0

Following issues are fixed in release 2.0 version.

#7 Support Async APIs.

#6 BeginTransaction API is no longer supported starting from Data.Net version 2.0

#5 Add capacity parameter for DataParameter class.

#4 Add support for .Net Framework 4.8.

#3 Set DBNull in SqlParameter object when a value is null.

#2 Object reference not set to an instance of an object.

Added .Net framework support 4.7.1 & 4.7.2 + Bug fix (Issue #1)

Issue fixed #1
Added test case "Query_Single_String()"
Added framwork support for .NET 4.7.1 & 4.7.2