include index.d.ts in the package

[franklixuefei]

fixes #53

This is the result of running npm pack locally:

src\index.js -> lib\index.js
npm notice
npm notice package: react-container-dimensions@0.0.0-development
npm notice === Tarball Contents ===
npm notice 2.4kB package.json
npm notice 945B  index.d.ts
npm notice 1.1kB LICENSE
npm notice 3.9kB README.md
npm notice 5.2kB lib/index.js
npm notice === Tarball Details ===
npm notice name:          react-container-dimensions
npm notice version:       0.0.0-development
npm notice filename:      react-container-dimensions-0.0.0-development.tgz
npm notice package size:  5.1 kB
npm notice unpacked size: 13.5 kB
npm notice shasum:        3fd760394a83bba466a227f024f49ad5cd5e94ee
npm notice integrity:     sha512-xO7Z31Tb+iQAb[...]ZfCjRAQemsOqA==
npm notice total files:   5
npm notice
react-container-dimensions-0.0.0-development.tgz