Change "Remember me" legend to "Keep me logged in", and implement it as a customizable string.

Fixes #346.

I also noticed that  [White label App Customization](https://github.com/SpiderOak/SpiderOakMobileClient/wiki/White-label-App-Customization) lacked explicit, concise instructions for implementing something as a customizable string(!), and added some.

This should also be merged to the master-ios-integration-branch.