Support Qiita:Team

Currently, Qiita:Team WebHook is not announced yet, but you can set webhook from Qiita:Team admin panel.

request examples here:
- [Webhookを使ってQiita:Teamのデータをバックアップする方法 - Qiita](http://qiita.com/suin/items/1acae96d34a9c51e20cd)
- [RequestBin - 1h9n7bc1](http://requestb.in/1h9n7bc1?inspect)

preview hooks:
![2014-06-21 09 12 16](https://cloud.githubusercontent.com/assets/167012/3347279/91f3eea0-f8da-11e3-8c8a-4491f754267e.png)
:+1: 
It's necessary to parse `base_url` because [URI.join](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/uri/rdoc/URI.html#method-c-join) takes string arguments :see_no_evil: