修改 Mix and Match 章节，完成 Interacting with Objective-C APIs 初稿

根据[上一个 PR](https://github.com/numbbbbb/the-swift-programming-language-in-chinese/pull/11) 修改 Mix and Match 章节，麻烦再试一下。

另外，修改了目录名，完成 [Interacting with Objective-C APIs](https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithObjective-CAPIs.html#//apple_ref/doc/uid/TP40014216-CH4-XID_26) 初稿。