LinkerPleaseInclude: Add CheckedTextView Switch

Add CheckedTextView and Switch to linker inclusions otherwise these bindings will fail in release builds.
Added Universal Windows Apps to TipCalc links in readme.md
Merge pull request #773 from kjeremy/v3.1

LinkerPleaseInclude: Add CheckedTextView Switch
Changed order or projects in TipCalc walkthrough
Weak ref change for #668 and #729
Merge pull request #780 from sequence/v3.1

Added Universal Windows Apps to TipCalc
Merge pull request #779 from kjeremy/v3.1

Weak ref change for #668 and #729
Merge branch 'v3.1' of https://github.com/MvvmCross/MvvmCross into v3.1