Introduced context class which has the Tip, CurrentBranch and Repo rather than setting properties down the line
No need to pass whole context object into Master Version finder
Tip is no longer set on context, it just delegates to Repository.Head.Tip
Fix broken test
Context.Tip now refers to CurrentBranch.Tip
Removing Tip helper on Context
Merge pull request #58 from JakeGinnivan/IntroduceContextObject

Introduced context class which has the Tip, CurrentBranch and Repo