Google Analytics command
add ga.js
refactoring
improve ga command

- `@hubot ga example.com`
- `@hubot ga example.com 2014-01-01`
- `@hubot ga example.com 2014-01-01 2014-01-31`
improve ga command (default from = last month)
Merge pull request #9 from bouzuya/ga

Google Analytics command