Added padding between img and the border

[willigro]

I needed apply a spacing between the image and the border.
So, i maked a propriety of padding (mRadiusPadding) that give this space, kept the image without cut.

[JcMinarro]

Hi @hdodenhof would be great to have this feature on the lib.
Were you able to review this PR?

[Magno-Ramos]

this looks great

[hdodenhof]

@willigro Thanks for your work on this!

I'll quote a statement a made earlier in #226:

I won't implement this directly as I'd like to keep this whole thing as simple to use as possible and part of that is keeping the number of options limited. I recommend using that workaround - you could also create a custom ViewGroup that completely wraps the magic.

I still think this is a niche use case that doesn't warrant the added complexity, esp. as there's a simple way to achieve the desired state in #133.