Simply fix the Image init problem with StringIO

Image __init__ with StringIO didn't work because StringIO class doesn't has split() method, which is called by webp determination. To solve this, a judgement is inserted before source.split to exclude that situation, thus it would only be called if source is not a StringIO instance.

To reproduce the bug:
```
import SimpleCV as scv
import StringIO

img = scv.Image('lenna')
buff = StringIO.StringIO
img.save(buff)

buff.seek(0)
new_image = scv.Image(buff)
```