Send Event to Event Hub

Implemented features:

• Added Event Hub Send Event REST API implementation (as for https://msdn.microsoft.com/en-us/library/azure/dn790664.aspx).

Refactoring:

• Corrected azure::core::errors::AzureError to include the std::io::Error and xml::BuilderError instead of their to_string() result.
• Changed UnexpectedResult enum of azure::core::errors::AzureError to UnexpectedHTTPResult which holds an instance of azure::core::errors::UnexpectedHTTPResult instead of a tuple for better field description.