implement file sources

If the file is not cached we have to get it from somewhere. For that I
implemented a webdav file source which gets the file from reva.

pkg/thumbnails/imgsource/imgsource.go

@@ -0,0 +1,33 @@
+package imgsource
+
+import "image"
+
+// Source defines the interface for image sources
+type Source interface {
+	Get(path string, ctx SourceContext) (image.Image, error)
+}
+
+// NewContext creates a new SourceContext instance
+func NewContext() SourceContext {
+	return SourceContext{
+		m: make(map[string]interface{}),
+	}
+}
+
+// SourceContext is used to pass source specific parameters
+type SourceContext struct {
+	m map[string]interface{}
+}
+
+// GetString tries to cast the value to a string
+func (s SourceContext) GetString(key string) string {
+	if s, ok := s.m[key].(string); ok {
+		return s
+	}
+	return ""
+}
+
+// Set sets a value
+func (s SourceContext) Set(key string, val interface{}) {
+	s.m[key] = val
+}