Clean up PostPreviewViewController code style.

Removes xib also.
Fixes check for scheduled posts.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1186-post-preview-auth

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
Merge pull request #1215 from wordpress-mobile/issues/1186-post-preview-auth

Issues/1186 post preview auth