first pass at supporting a default base URL

this allows remote images to be specified as relative URLs, relative to
the `DefaultBaseURL` field.  Fixes #15.

data.go

@@ -199,7 +199,7 @@ type Request struct {
 // 	http://localhost/100x200,r90/http://example.com/image.jpg?foo=bar
 // 	http://localhost//http://example.com/image.jpg
 // 	http://localhost/http://example.com/image.jpg
-func NewRequest(r *http.Request) (*Request, error) {
+func NewRequest(r *http.Request, baseURL *url.URL) (*Request, error) {
 	var err error
 	req := new(Request)
 
@@ -220,6 +220,10 @@ func NewRequest(r *http.Request) (*Request, error) {
 		req.Options = ParseOptions(parts[0])
 	}
 
+	if baseURL != nil {
+		req.URL = baseURL.ResolveReference(req.URL)
+	}
+
 	if !req.URL.IsAbs() {
 		return nil, URLError{"must provide absolute remote URL", r.URL}
 	}