Test code refactor #63

Use mocha for feature-specific unit tests

Add test coverage report via istanbul

test/leak/leak.sh

@@ -0,0 +1,8 @@
+if ! type valgrind >/dev/null; then
+  echo "Please install valgrind before running memory leak tests"
+  exit 1
+fi
+
+curl -O https://raw.githubusercontent.com/jcupitt/libvips/master/libvips.supp test/leak/libvips.supp
+cd ../../
+G_SLICE=always-malloc G_DEBUG=gc-friendly valgrind --suppressions=test/leak/libvips.supp --suppressions=test/leak/sharp.supp --leak-check=full --show-leak-kinds=definite,indirect,possible --num-callers=20 npm test