Add podspec.

(PR for #4)

You have to make a tag `0.1.0` for this repository.

```bash
$ git checkout master
$ git tag 0.1.0
$ git push --tags
```

Then, register to CocoaPods.

```bash
$ pod trunk register YOUR_EMAIL NAME
```

After email validation, push it to CocoaPods.

```bash
$ pod trunk push
```