New Intercom's API

Update Intercom-php with the newest API of Intercom

http://doc.intercom.io/api/

### Changelog

- The new common endpoint used by ``User`` and ``Event`` clients is ``https://api.intercom.io``
- For **POST**, **PUT**, **PATCH** and **DELETE** request, body is sent in ``json`` parameter instead of ``body``
- Merge ``Client\User::create()`` and ``Client\User::update()`` into ``Client\User::createOrUpdate()``
- ``Client\User::createOrUpdate()`` handle bulk if an array is given.
- ``Object\User::customData`` become ``Object\User::customAttributes``
- ``PaginatedResponse::nextPage`` become ``PaginatedResponse::nextUrl``.
- ``UserSearch::perPage`` default is now 50 instead of 100
- ``UserSearch::tagName`` removed
- ``UserSearch::sort`` removed 
- ``UserSearch::segmentId`` added