Hide file format especific savers. Add generic image saver.

Misc fixes under OSX.

src/nvtt/tests/imperativeapi.cpp

@@ -29,7 +29,10 @@
 int main(int argc, char *argv[])
 {
 	nvtt::CompressionOptions compressionOptions;
+	compressionOptions.setFormat(nvtt::Format_BC1);
+
 	nvtt::OutputOptions outputOptions;
+	outputOptions.setFileName("output.dds");
 
 	nvtt::Context context;
 	nvtt::Texture texture = context.createTexture();