Merge pull request #2 from logankilpatrick/patch-1

Update README.md

add api = nullptr;

Update version to 0.3.1

Add new files and change version

Update version

Update CHANGELOG.md

Add asynchronously support

Call function createCompletion asynchronously

Change ApiHelper with asynchronously support

Change ApiHelper to class with async support...

Add std::future return type for asynchronously

Add Response class and struct ServerResponse

Add Response class and std::future return type

Update ApiHelper.cpp