#47 Add option to not generate image for signalp

[daveriley]

It would be great if we could have the image generation in signalp be optional. The images take up a significant amount of space and are probably not very useful in most cases. At present if the image type parameter is left blank the component fails because it passes and empty -graphics option to signalp. Additionally, there is a 'compress graphics' step that would need to be removed completely. At present since there is no way that I know of to add optional parameters it might be useful to add a second component called signalp_no_images that simply leaves out the graphics option and the compress step.