.gitignore: ignore all files inside /lib but keep the folder [fixes #1] - this way the initial `npm run build` doesn't fail with:
> react-boilerplate@0.2.0 build /Users/paul/react-boilerplate
> jsxc src/ lib/
ERROR: "lib/" must exist
Merge pull request #4 from paulwittmann/master

.gitignore: ignore all files inside /lib but keep the folder [fixes #1] ...