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 the custom parameters accessor as 'params'
Add tests for the custom parameters feature
Document the custom parameters feature in the README
Merge branch 'feature/params' into develop (fixes #6)
Move advanced documentation from the README to the wiki
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
Raise errors occurring during initialization
Prevent declaring a column with block and no attribute
Support declaring columns as optional
Reorganize documentation links in README
Bump version to 0.2.5