Added optional functionality for the user to enter a custom gulp - issue #10

[Ross-Gibson]

Overview
I've added a switch in the portions section of the preferences page, enabling the user to toggle custom gulp sizes. This switch if off by default, so the user experience remains consistent.

Enabling custom gulps adds a third option when the add button is pressed from the drink tab. This option presents a modal view controller, and asks the user to enter the size of the custom gulp.

The feature conforms to the existing model supporting both units of measure, and has been translated to Italian (via Google Translate).

The project is iOS 9 only, so I have taken advantage of UIStackView alongside Auto-Layout to manage the UI. The custom gulp button is positioned above the small and big gulp buttons, but will not overlap with the existing percentage label.

[andreamazz]

Hi Ross
Thank you for the PR, this is pretty great. I'm going to merge this in the next few days, there are a couple of things that I would change, but nothing major, I'll do it once it's merged.
Only one question... I see you disabled the View controller controlled status bar, is there any specific reason behind it? I needed that to be on since the onboarding should be presented without status bar.

[Ross-Gibson]

Hi Andrea,

Thanks for taking the time to review my contributions. Gulps is a nice project to work on, and very well structured.

My reason for the status bar change is due to the presentation of the "Custom Gulp" view controller. To keep with the style of the app I've presented this view controller modally, and set the navigation bar colour to blue (copying the design from your onboarding). Without doing so the status bar was being hidden.

It's my fault for not checking the onboarding against the App Store build, and I was unaware that the status bar was originally hidden in the onboarding. More than happy to resolve this, and take a look at any other things you'd like changed.

~ Ross