Added support for multi targetting in NuGet package (#75)

Author: hugener

ILRepack.Lib.MSBuild.Task/build/ILRepack.Lib.MSBuild.Task.nuspec

@@ -19,6 +19,7 @@
     </metadata>
     <files>
         <file src="ILRepack.Lib.MSBuild.Task.dll" target="build"/>
-        <file src="ILRepack.Lib.MSBuild.Task.targets" target="build"/>
+		<file src="ILRepack.Lib.MSBuild.Task.targets" target="build"/>
+		<file src="ILRepack.Lib.MSBuild.Task.targets" target="buildCrossTargeting"/>
     </files>
 </package>