This commit introduces two new methods that every
AMo compliant object must implement. Below are the
default implementations along with the implied
interface contract.

  # Returns an Enumerable of all (primary) key
  # attributes or nil if new_record? is true
  def key
    new_record? ? nil : [1]
  end

  # Returns a string representing the object's key
  # suitable for use in URLs, or nil if new_record?
  # is true
  def to_param
    key ? key.first.to_s : nil
  end

1) The #key method

Previously rails' record_identifier code, which is
used in the #dom_id helper, relied on calling #id
on the record to provide a reasonable DOM id. Now
with rails3 being all ORM agnostic, it's not safe
anymore to assume that every record ever will have
an #id as its primary key attribute.

Having a #key method available on every AMo object
means that #dom_id can be implemented using

  record.to_model.key # instead of
  record.id

Using this we're able to take composite primary
keys into account (e.g. available in datamapper)
by implementing #dom_id using a newly added

  record_key_for_dom_id(record)

method. The user can overwrite this method to
provide customized versions of the object's key
used in #dom_id.

Also, dealing with more complex keys that can
contain arbitrary strings, means that we need to
make sure that we only provide DOM ids that are
valid according to the spec. For this reason, this
patch sends the key provided through a newly added

  sanitize_dom_id(candidate_id)

method, that makes sure we only produce valid HTML

The reason to not just add #dom_id to the AMo
interface was that it feels like providing a DOM
id should not be a model concern. Adding #dom_id
to the AMo interface would force these concern on
the model, while it's better left to be implemented
in a helper.

Now one could say the same is true for #to_param,
and actually I think that it doesn't really fit
into the model either, but it's used in AR and it's
a main part of integrating into the rails router.

This is different from #dom_id which is only used
in view helpers and can be implemented on top of a
semantically more meaningful method like #key.

2) The #to_param method

Since the rails router relies on #to_param to be
present, AR::Base implements it and returns the
id by default, allowing the user to overwrite the
method if desired.

Now with different ORMs integrating into rails,
every ORM railtie needs to implement it's own
#to_param implementation while already providing
code to be AMo compliant. Since the whole point of
AMo compliance seems to be to integrate any ORM
seamlessly into rails, it seems fair that all we
really need to do as another ORM, is to be AMo
compliant. By including #to_param into the official
interface, we can make sure that this code can be
centralized in the various AMo compliance layers,
and not be added separately by every ORM railtie.

3) All specs pass