Hide Apptentive Branding remote configuration option.

 * Added ability to remotely hide Apptentive branding in your app via the Apptentive dashboard, contingent upon your account plan.
 * Added `initiallyHideBranding` property, which hides Apptentive branding in the time prior to the app's initial configuration being retrieved.
 * Removed `showTagLine` property, which has been replaced by `initiallyHideBranding` and the remote configuration.
Modern Objective-C automatically synthesizes a property's backing ivar: `_initiallyHideBranding`. It was done manually for `showTagLine`.