[#655] Added `included` tracking to Model::Search
[#655] Extracted import Rake task into `import:all` and `import:model`
Refactored the `import:model` and `import:all` Rake tasks

Related: #656, #652

Closes #655
Removed the manual Tire integration tracking for models

The `dependents` tracking in `included` isn't really needed. We can always ask:

    Article.respond_to?(:tire)

Or:

    Article.included_modules.include?(Tire::Model::Search)

Revert "[#655] Added `included` tracking to Model::Search" (6eac80f34126fb6c32d687d06840978adf243266)

Related: #655, #652
CC: @jwaldrip
Refactored the `import:all` task to not use the manual Tire integration tracking

* This builds on changes in d467fc5
* Uses the `classify` and `constantize` from ActiveSupport
* Skip any model without Tire integration

Related: #655, #652
[#656] Added adapter specific importing for different ORMs/OHMs/ODMs/OxMs
[#656] Refactored model-specific importing into a set of strategies

* Cleaned up the code, use `Tire::Import::Strategy` to wrap strategy-choosing logic
* Implemented each strategy as a class with the `import` method
* Used strategy in `MyModel.import`
* Updated documentation

Related: #656, #652, #272, #576, #530

Closes #656