Add "powered by ODK" to footer

[lognaturel]

Should be small and tasteful, link to website.

[alyblenkin]

Here are a few ideas! I'm leaning towards 1 and 2, but curious to know what you all think.

[sadiqkhoja]

Are we thinking to make it a license requirement for clients/host applications not to hide it?

[lognaturel]

I don't think so. I don't think it's actually a valid/enforceable addition to Apache 2.0. My feeling is that host applications run by folks who understand that this kind of minimal branding is helpful to the long-term viability of a component they're relying on would tend to leave it regardless and folks who don't know/care would remove it no matter what. I think a lot of likely integrating organizations also understand that it helps their users when those users have a way of knowing the details of the software they're using and how they can get deeper help.

[sadiqkhoja]

done in #186