Remove rubygems and bundler require.

This means that local development may now require a 'bundle exec' if
you are using Bundler, but that MoSQL will run fine in environments
where the gems are made available through some other means, and
bundler is not available.
Refactor the collections YAML format to be more flexible.
Support both the new and the old schema format
Allow renaming attributes during import.

- Update the README to provide more details about the new configuration format.
- Illustrate the possibility to rename attributes during the import.
- Make it clear that the old configuration format is still supported.
Update the README slightly for grammar and clarity.
Merge branch 'pr/15'
Fix bulk_upsert's fallback case, and add a test.

closes #17.
Refactors the collections YAML format to be more flexible for upcoming dot syntax reading
Add callbacks feature

Allow the user to specify callback classes on a per collection level.
When defined these classes allow to trigger custom actions after an
upsert of a delete operation has been propagated to the SQL database.
Add import_order meta attribute

This is really useful when playing with callbacks during the initial
import of the database.
Update README

Document the callback feature.
Fix errors with ruby 1.8

Make the code work also with ruby 1.8
Add pre-upsert hook