public methods and variables to be able to use picfit as library

[theromis]

I'm using picfit as a library (same way as django-sorl-thumbnail) to generate thumbnails on remote GCS storage, after last update my code stopped working. So I think this changes can be useful not only for me :)

func processImage(processor *picfit.Processor, qs map[string]interface{}, storeKey string, async bool) (*image.ImageFile, error) {

    file := &image.ImageFile{
        Key:     storeKey,
        Storage: processor.DestinationStorage,
        Headers: map[string]string{},
    }

    var filepath string

    var err error

    // URL provided we use http protocol to retrieve it
    filepath = qs["path"].(string)
    if !processor.SourceStorage.Exists(filepath) {
        return nil, failure.ErrFileNotExists
    }

    file, err = image.FromStorage(processor.SourceStorage, filepath)
    if err != nil {
        return nil, err
    }

    parameters, err := processor.NewParameters(file, qs)
    if err != nil {
        return nil, err
    }

    file, err = processor.Engine.Transform(parameters.Output, parameters.Operations)
    if err != nil {
        return nil, err
    }