Implement track/stream

* Add Streamer class
* Use FactoryGirl for factories
* Add sha256 field to tracks
* Add mime_type field to tracks

lib/streamer.rb

@@ -0,0 +1,12 @@
+class Streamer
+  def initialize(file)
+    @file = file
+  end
+
+  def each
+    f = File.new(@file, 'r')
+    while data = f.read(4096) do
+      yield data
+    end
+  end
+end