Add DownloadURL field to RepositoryContent struct

The RepositoryContent struct doesn't quite map to the API because it is
missing a download link.  This link is very useful because while the API
doesn't allow large files to be downloaded, the link allows the raw
files to be downloaded outside the API.

github/repos_contents.go

@@ -19,16 +19,17 @@ import (
 
 // RepositoryContent represents a file or directory in a github repository.
 type RepositoryContent struct {
-	Type     *string `json:"type,omitempty"`
-	Encoding *string `json:"encoding,omitempty"`
-	Size     *int    `json:"size,omitempty"`
-	Name     *string `json:"name,omitempty"`
-	Path     *string `json:"path,omitempty"`
-	Content  *string `json:"content,omitempty"`
-	SHA      *string `json:"sha,omitempty"`
-	URL      *string `json:"url,omitempty"`
-	GitURL   *string `json:"git_url,omitempty"`
-	HTMLURL  *string `json:"html_url,omitempty"`
+	Type        *string `json:"type,omitempty"`
+	Encoding    *string `json:"encoding,omitempty"`
+	Size        *int    `json:"size,omitempty"`
+	Name        *string `json:"name,omitempty"`
+	Path        *string `json:"path,omitempty"`
+	Content     *string `json:"content,omitempty"`
+	SHA         *string `json:"sha,omitempty"`
+	URL         *string `json:"url,omitempty"`
+	GitURL      *string `json:"git_url,omitempty"`
+	HTMLURL     *string `json:"html_url,omitempty"`
+	DownloadURL *string `json:"download_url,omitempty"`
 }
 
 // RepositoryContentResponse holds the parsed response from CreateFile, UpdateFile, and DeleteFile.