Add assertion checks for API key usage

[msmollin]

We should be more aggressive about checking for API key usage in the SDK and utilize something like Cocoa's assert() to avoid assertions being compiled into production code.

[msmollin]

@sarahlensing i think this is done now?

[msmollin]

Or did we only add this to the sample app?

[sarahsnow1]

We only added it to the sample app so we should keep this open and update MapzenSearch, etc.

[msmollin]

Looked at this a little bit and we do kinda already check for this, but just in MZMapViewController https://github.com/mapzen/ios/blob/master/MapzenSDK/MZMapViewController.swift#L1082

And it throws, it doesn't assert the api key missing. Since I can't change that API do you think I should just add an assert into that? Or should I have the other places throw as well?

[msmollin]

After discussion we're going to make this assert and change the call to not throw anymore and deprecate all the loadStyle* functions and create new ones that no longer throw as well. We'll release this as part of 1.1.0.