Merge pull request #282 from morgoth/unify-calling-include_attribute

Unify way of calling include_attribute? in attibutes and association methods
Change gem.homepage to new location
Merge pull request #285 from bquorning/patch-1

Change gem.homepage to new location
fix travis links

The existing links work, but redirect.
Merge pull request #289 from teeparham/patch-2

fix travis links
Added support for :only and :except methods.

It is possible now to filter returned attributes and associations by:

  UserSerializer.new(user, only: [:first_name, :last_name])
  UserSerializer.new(user, except: :first_name)