Adds @3x support

Because @3x is something we'll all need now.
I've also added automatic path extension support and retina suffix.

This 3 examples will import the same image:
`[OLImage imageNamed:@"file@3x.gif"]`
`[OLImage imageNamed:@"file@3x"]`
`[OLImage imageNamed:@"file"]`

Makes working with OLImageView a lot more fun!