Source configuration uses owner instead of user

"Owner" is the name GitHub uses in its API to reference the owner of a
repository. "User" is not correct when the owner is an organization.

This removes some confusion I've encountered over how to configure the
resource while maintaining backwards compatibility.

resources.go

@@ -1,9 +1,12 @@
 package resource
 
 type Source struct {
-	User       string `json:"user"`
+	Owner      string `json:"owner"`
 	Repository string `json:"repository"`
 
+	// Deprecated; use Owner instead
+	User string `json:"user"`
+
 	GitHubAPIURL     string `json:"github_api_url"`
 	GitHubUploadsURL string `json:"github_uploads_url"`
 	AccessToken      string `json:"access_token"`