Implement check for Organization membership

There are more methods of checking members of an organizations, especially checking public memberships, which we already have implemented.

However, there is [this](https://developer.github.com/v3/orgs/members/#get-organization-membership) method which we haven't implemented and is quite useful.

Basically, if the logged in user is a member of that organization, he can check another member's type of membership regardless if that member has publicized it or not.

It could be something like `Organization.membership(String memberLogin):Membership`, and `Membership` interface would describe the json returned by the above mentioned endpoint.

**Important**
If implemented, it should be thoroughly documented, so that no one mistakes it for the public membership endpoints.