Added "Functions" Chapter. Rename some files.

Added "Functions" Chapter,
Rename some files.
Each chapter has one corresponding file.
merged
Added "Functions" Chapter. Rename some files.

Added "Functions" Chapter,
Rename some files.
Each chapter has one corresponding file.
merged
Merge branch 'translate-swift' of https://github.com/honghaoz/the-swift-programming-language-in-chinese into translate-swift
Rename strings_and_characters.md to Strings_and_Characters.md
completed "Defining and Calling Functions"
Merge pull request #14 from honghaoz/translate-swift

Added "Functions" Chapter. Rename some files.