Clean Specification::DSL documentation.

Removed accessor methods and other minor clean ups.
Port CocoaPods improvements to Bacon spec helper

Also add some minor improvements.
[RootAttributesAccessors] Add module to handle root attributes.

With this module there is no need to use meta programming for root attributes.
[Specification::DSL] Clean up the docs.
[Specification] Store attributes values in a simple hash.

- Paves the support for a logic less format like YAML.
- Favors composition over meta programming.
- Reduces the responsibility of the DSL module which now, is only concerned
  about storing the attributes and not about accessing them.
- Deprecates the needed of the `Specification#activate_for_platform` call
  in favor of the use of the `Specification::Consumer` class.
[Specification::DSL] Improve exclude_files default.
[Specification::Consumer] Complete and refine implementation.
[Spec::PlatformProxy] Move to Spec::DSL::PlatformProxy

Also base the implementation on method missing, which ensures that all the specification attributes have been loaded.
[Specification] Extract Spec::DSL::Deprecations
[Specification] store values in a hash and clean up implementation.
[Platform] add #inspect.
[Specification::DSL] Remove last bits of meta programming.

Completes the support for storing the data of the specification in a
hash and for the recently introduced consumer class.
Propagate changes to the Specification class.
[SpecHelper] Fine tune bacon support.
[Specification] Store the values of the attributes by string.

The use of a symbol would lead to a non optimal YAML implementation.
[Specification] Store the keys of the hashes as a String.

The keys are converted to a symbol by the accessors.
[Specification] improve #==
[Specification] take into account the callbacks in #==
[Specification] Complete YAML support
[Specification] Store the platform information in a YAML friendly way.