Added ExecuteAsync overloads that return Task<T>. fixes #366
Renamed methods to conform to TPL standards and added overloads for GET and POST specific usage and added overloads without CancellationToken
Fixed indentation
Throw an exception when ExecuteTaskAsync fails
Added unit tests for task-based async methods
Added parameter checks for NULL values
Merge pull request #367 from ErikSchierboom/issue-366

Add Task<T> overloads to ExecuteAsync