Added debug attributes and fixed a few issues

• Error reports now include debug attributes
• BacktraceClient.OnClientReportLimitReached event handlers will now take BacktraceReport as a parameter
• Refactoring of JSON Data code.
• .NET 4.5 example with async task usage
• Better stack trace analysis
• Fix: Reporting unhandled application exceptions now uses SendAsync and proper TLS 1.2 support

Added asynchronous send and streaming attachments from file

• BacktraceClient now supports an asynchronously SendAsync method that works with async task
• For .NET Framework 4.5 and .NET Standard 2.0, BacktraceClient now streams file attachment content directly from disk via SendAsync method.
• AfterSend event parameter changed. Now AfterSend event require BacktraceResult parameter, not BacktraceReport,
• Send and SendAsync method now returns BacktraceResult with information about report state,
• OnServerResponse now require BacktraceResult as a parameter.