Support enclosing the import process in a transaction block
Use real ActiveRecord models for specs
Expose any raised errors when using transactions
Add full specs for transaction mode
Rename 'transaction' to 'transactional'
Update README with info about transactional mode
Rename use_transaction? to transactional?
Support setting an importer instance as transactional

Conflicts:
	lib/active_importer/base.rb
Merge branch 'feature/transactions' into develop

Conflicts:
	lib/active_importer/base.rb