Add WelcomeViewController.swift class into the project.
Add UILabels and UIImage into the WalkthroughStoryboard and their
IBOutlets.

Design improvements : Change background and button colors, make
button width equals, reduce logo width and add content edge insets to
the RoundedButton. Navigation bar is hidden and the status bar is
white on the welcome screen.

Tuleap: #1399
Change-Id: Id5a373f966d0a70522764c0773e6105f32481754
Reviewed-by: Romain Bertozzi <romain.bertozzi@savoirfairelinux.com>