Two new methods

Hello, 
at first thank you for your cool library - I use it in one of my projects and it save me many days

I've implemented two more functions from Magick Wand API
1. ResizeWithFilter - same as Resize(), but you can specify filter  used in resizing and blur (Just like in PHP API)
2. GetImageBlob - I use it to outpt image directly to client, without saving it to file and reading again

Also I've added tests for each function and run go fmt on it