Option to use truncated signature (#88)

* CircleCI

* Fix docs resize_type -> resizing_type (#91)

As noted in my issue...

* Option to use truncated signature

* Don't use 0 as no-limit for SignatureSize

crypt_test.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"encoding/base64"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSignatureFor(t *testing.T) {
+	oldSignatureSize := conf.SignatureSize
+	base64Signature := func(x string) string { return base64.RawURLEncoding.EncodeToString(signatureFor(x)) }
+	conf.Key = []byte("test-key")
+	conf.Salt = []byte("test-salt")
+	assert.Equal(t, "dtLwhdnPPiu_epMl1LrzheLpvHas-4mwvY6L3Z8WwlY", base64Signature("asd"))
+	assert.Equal(t, "8x1xvzxVqZ3Uz3kEC8gVvBfU0dfU1vKv0Gho8m3Ysgw", base64Signature("qwe"))
+	conf.SignatureSize = 8
+	assert.Equal(t, "dtLwhdnPPis", base64Signature("asd"))
+	assert.Equal(t, "8x1xvzxVqZ0", base64Signature("qwe"))
+	conf.SignatureSize = oldSignatureSize
+}