Mapper .attribute method now accepts a block

[groyoh]

Mapper can now define attributes using a block, e.g.:

def PostMapper < Yaks::Mapper
  attribute :full_name do
    "#{object.first_name} #{object.last_name}"
  end
end

Resolves #15.

[plexus]

I'll let Travis finish for good measure. Also gives you some time if you like to still add something to the README, you can just push again to the same branch.

[plexus]

Great work! Implementation, tests, README, all look really good. One thing I forgot to mention is to update the CHANGELOG, but I tend to review things before I release a new gem so that's not a big problem.