Handle escaped quotes for GraphQL String Values

This also includes a new type for Value String.

The tests fail now, although it parses successfully. I'll use a pretty
printer in next commit so that it's easier to spot the differences.
Onces this is working I'll add the rest of the escaped characters.

Data/GraphQL/AST.hs

@@ -66,12 +66,14 @@ data Value = ValueVariable Variable
            -- GraphQL Float is double precison
            | ValueFloat Double
            | ValueBoolean Bool
-           | ValueString Text
+           | ValueString StringValue
            | ValueEnum Name
            | ValueList ListValue
            | ValueObject ObjectValue
              deriving (Eq,Show)
 
+newtype StringValue = StringValue Text deriving (Eq,Show)
+
 newtype ListValue = ListValue [Value] deriving (Eq,Show)
 
 newtype ObjectValue = ObjectValue [ObjectField] deriving (Eq,Show)