Add nullability annotations to public API

[nolanw]

As mentioned in #105.

Nullability annotations were introduced in Xcode 6.3 (which included iOS 8.3). I'm not sure what the minimum supported Xcode version is in these parts, but if 6.3 is a reasonable minimum then we shouldn't need any #define NS_ASSUME_NONNULL_BEGIN etc. to pacify older versions of the dev tools. Let me know if I've guessed wrong here.

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[nolanw]

I signed it!

[googlebot]

CLAs look good, thanks!

[morganchen12]

LGTM

[morganchen12]

@mcdonamp please merge when you get a moment

[asciimike]

Two public API changes

[asciimike]

We don't want this nullable as nil is not a valid location.

[asciimike]

Note that the internal API setLocationValue:forKey:withBlock: (https://github.com/firebase/geofire-objc/blob/master/GeoFire/Implementation/GeoFire.m#L84) allows location to be nil (when we delete things), but the public API probably shouldn't allow this.

[nolanw]

Ah yep, I was basing it off the internal API.

[asciimike]

Same comment as above, probably not nullable.

[asciimike]

@nolanw, @morganchen12 Sorry about the delay. One minor change on the public surface; otherwise, LGTM. Thanks for doing this!

[nolanw]

Not nullable it is!

[asciimike]

LGTM

[asciimike]

@morganchen12, do you have permissions to release, or do you need me to?

[morganchen12]

I don't have release permissions, so please make a new release when you have the time.

Thanks!

[asciimike]

Published in v2.0.1.