Fix CS0649 warnings about unused fields (#83)
Fix CS0649 warnings about unused fields (#83)

- JavascriptParser.cs(30,31): warning CS0649:
  Field 'JavaScriptParser.Context.MarkerStack' is never assigned to,
  and will always have its default value null
- JavascriptParser.cs(19,24): warning CS0649:
  Field 'JavaScriptParser.Context.LastCommentStart' is never assigned
  to, and will always have its default value 0
sebastienros/esprima-dotnet,sebastienros/esprima-dotnet