It was not covered, probably because it was hard to do something
generic. I choose to duplicate some tests instead, so that things stay
relatively simple.

- fixes platform's COMMENT ON statement support
- implements reverse engineering for column comments
- marks boolean column type commented for distinction

fixes #2182

cover in test suite that platform throws exception on generating inline column comment SQL if unsupported

fixes #2270

Unit tests currently only cover SQLite, MySQL and PostgreSQL.

Tests for other platforms have been marked as incomplete.

This coherent with what is done for Table. All platform specific things
are grouped into an options array. Eventually flags should be migrated
into options as well.

Support for Partial Indexes was available in Doctrine 1 following
http://www.doctrine-project.org/jira/browse/DC-82. This commit
reintroduce support for Doctrine 2. We use the same syntax with an
optionnal "where" attribute for Index and UniqueConstraint.