Added sections and articles for help center
Merge with Interfaces
Added Async methods on HelpDesk sections articles
Remove nullabe designation from DueAt. It causes a build error, and is unnecessary as string is a nullable type.
Add HelpCenter Category models
Correct HelpCenter.Categories namespace
Add HelpCenter Categories requests
Add locale and tests for HC categories
Merge irq's article and section features
Formatting fix
Merge branch 'master' of https://github.com/peteallen/ZendeskApi_v2 into peteallen-master
got branch to compile