fix test references

update references for unit tests to reference new core and fonts projects. all tests except the public api scanner tests now run successfully.

src/UglyToad.PdfPig.Tests/Parser/Parts/FileHeaderParserTests.cs

@@ -1,8 +1,8 @@
 namespace UglyToad.PdfPig.Tests.Parser.Parts
 {
     using System;
-    using Exceptions;
     using Logging;
+    using PdfPig.Core;
     using PdfPig.Parser.FileStructure;
     using Xunit;