add imageproxy-sign tool for calculating signatures

it's a little bit rough, but seems to work pretty well.

Ref #145

cmd/imageproxy-sign/main_test.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestParseURL(t *testing.T) {
+	tests := []struct {
+		input, output string
+	}{
+		{"/", "/#0x0"},
+
+		// imageproxy URLs
+		{"http://localhost:8080//http://example.com/", "http://example.com/#0x0"},
+		{"http://localhost:8080/10,r90,jpeg/http://example.com/", "http://example.com/#10x10,jpeg,r90"},
+
+		// remote URLs, with and without options
+		{"http://example.com/", "http://example.com/#0x0"},
+		{"http://example.com/#r90,jpeg", "http://example.com/#0x0,jpeg,r90"},
+
+		// ensure signature values are stripped
+		{"http://localhost:8080/sc0ffee/http://example.com/", "http://example.com/#0x0"},
+		{"http://example.com/#sc0ffee", "http://example.com/#0x0"},
+	}
+
+	for _, tt := range tests {
+		want, _ := url.Parse(tt.output)
+		got := parseURL(tt.input)
+		if got.String() != want.String() {
+			t.Errorf("parseURL(%q) returned %q, want %q", tt.input, got, want)
+		}
+	}
+}