Pdf Import Rewrite

Pdf Import rewritten to greatly improve QR Code detection, reduce
reliance on iTextSharp and improve thumbnails. Fixes #50

Disco.Services/Expressions/IExpressionPart.cs

@@ -0,0 +1,15 @@
+using System.Collections;
+
+namespace Disco.Services.Expressions
+{
+	public interface IExpressionPart
+	{
+        string RawSource { get; set; }
+		string Source { get; set; }
+        bool ErrorsAllowed { get; set; }
+        bool ParseError { get; }
+        string ParseErrorMessage { get; }
+		bool IsDynamic { get; set; }
+		object Evaluate(object ExpressionContext, IDictionary Variables);
+	}
+}