Relationships: Improve docs and replace strings with constants

[spawnia]

When specifying relationships via hasOne and hasMany, i do not like that i have to rely on those magic strings - and could easily make a mistake typing them out.

I propose replacing them by static constants, which are available on the JsonApi class. This way, it is possible to use in it the code as following:

import JsonApi from 'devour-client'

...
myApi.define(
  'post',
  {
    name: '',
    comments: {
      jsonApi: JsonApi.HAS_MANY,
      type: 'comments',
    }
  },
)

Either way, it would be nice if the section Relationships clearly listed the available types of relationships and provided an example for it.

I would be willing to create a pull request if you think this change is a good idea.