put schema in folder module, as it will contain more things
extract MAQL strings from columns.py, and refactor the column classes. schema.maql contains all the MAQL that will be needed in the project
initialize the migration module with basic action: add a column
add get_drop_maql method to Column, in order to drop a column
rename migrations module to migration without an s
add dataset class method to retrieve a synchronize statements, and add the synchronize statement in schema module
add a MigrationFailed exception
add a migration.chain module, initialized with MigrationChain class
add a test file to test the migration module
add dataset method has_attribute and has_fact to query GD API
change methods names in actions (execute->get_maql) and factorize chain code
add tests to check AddColumn and DeleteColumn (success) and modify department example to do so
little bugfixes in columns.py and put CASCADE statement after a drop for attribute or fact (if not, it wont work)
test a migration sequence, and bugfixes in migration class
add template MAQL to drop / alter columns
add TEMPLATE_DROP attribute in classes: Date, Time, Label, Reference
add get_alter_maql method to columns
add SYNCHRONIZE_PRESERVE, that is required to preserve data during a migration
use preserve=True in migration chain
add migration/utils.py file, to gather utility functions for migration