create sample app to test cocoapods integration

[jingwei-huang1]

• Create Package.swift to prepare swift package manager integration
• Move LayoutKit to Source folder to prepare swift package manager integration
• Create test project by using LayoutKitSampleApp
• Use cocoapods to integrate LayoutKitSampleApp
• Update build script to test cocoapods integration

[jingwei-huang1]

@nicksnyder @staguer This PR is hard to review and I moved this branch to LayoutKit from my folk repo. So please check it out and review locally.

[staguer]

The top level directories include:
Example
ExampleLayouts
Source
LayoutKitTests

This makes it seem like examples and tests aren't source code. I mentioned this in an earlier comment.

What do you think about structuring the directories like so:
Source/ExampleApp
Source/ExampleLayouts
Source/LibraryTests
Source/Library

[staguer]

If you haven't yet, please verify that this automated test does indeed fail if a mistake is introduced.

[staguer]

Please add a comment explaining why the order Derived Data clearing is important.

[nicksnyder]

Won't have a chance to pull and test, but I didn't see any obvious problems. As Sergei said, just make sure that this does indeed fail if a error is introduced in the podspec