Build Cleanup

* Remove legacy NAnt-based build files
* Remove all references to NET_2_0 and later framework defines
  * Currently, we have no code distinctions between these frameworks
* <TargetFrameworkVersion>v4.5.2</TargetFrameworkVersion>
* Revise Travis CI mono version

Author: McNeight

.travis.yml

@@ -1,7 +1,7 @@
 language: csharp
 mono:
   - latest
-  - 3.12.1
+  - 3.12.0
   - 2.10.8
 solution: ICSharpCode.SharpZipLib.sln
 #install: