Support generating capture URL

This includes the basic captureas package, including the API client and
a Capture() func that generates capture URLs.

capture.go

@@ -0,0 +1,44 @@
+package captureas
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"net/url"
+)
+
+type CaptureParams struct {
+	URL    string `field:"url"`
+	Width  int    `field:"w"`
+	Height int    `field:"h"`
+}
+
+func (c *Client) Capture(cp *CaptureParams) string {
+	u, err := url.Parse(c.Config.BaseURL)
+	if err != nil {
+		return ""
+	}
+
+	// Build query string
+	q := u.Query()
+	q.Add("url", cp.URL)
+	if cp.Width > 0 {
+		q.Add("w", fmt.Sprintf("%d", cp.Width))
+	}
+	if cp.Height > 0 {
+		q.Add("h", fmt.Sprintf("%d", cp.Height))
+	}
+	u.RawQuery = q.Encode()
+
+	// Calculate token
+	h := md5.New()
+	ue, _ := url.QueryUnescape(u.RawQuery)
+	io.WriteString(h, "?"+ue)
+	io.WriteString(h, c.Secret)
+	token := hex.EncodeToString(h.Sum(nil))
+
+	u.Path = "/" + c.Username + "/" + token + "/png"
+
+	return u.String()
+}

captureas.go

@@ -0,0 +1,25 @@
+// Package captureas provides the binding for the Capture.as API
+package captureas
+
+import (
+	"code.as/core/api"
+)
+
+const (
+	apiURL    = "https://capture.as"
+	userAgent = "go-captureas"
+)
+
+type Client struct {
+	*as.Client
+	Username string
+	Secret   string
+}
+
+func NewClient(username, secret string) *Client {
+	return &Client{
+		Client:   as.NewClient(as.NewClientConfig(apiURL, userAgent)),
+		Username: username,
+		Secret:   secret,
+	}
+}