Remove segue .showGoogle

[mindgraffiti]

Fixes #218
Ref. #182

This PR removes all references to the segue .showGoogle and programmatically navigates the user to the SignupGoogleViewController . There are 3 areas where the segues have been removed:

1. WPiOS sign up
2. WPiOS sign in
3. WCiOS sign in

To Test - WPiOS

1. Check out this branch
2. rake dependencies and ensure there are no errors
3. Build and run (there should be no errors)
4. Visit the WPiOS PR to run the changes: Remove the .showGoogle segue and replace with programmatic nav  WordPress-iOS#13720

To Test - WCiOS

1. Check out this branch
2. rake dependencies
3. Build and run (there should be no errors)
4. Visit the WCiOS PR to run the changes: [Auth] Remove the .showGoogle segue and navigate programmatically woocommerce/woocommerce-ios#2031

[ScoutHarris]

Tested with wordpress-mobile/WordPress-iOS#13720.