Support for Partial Indexes for PostgreSql and Sqlite

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.

It is unit-tests covered and documented in manual.
Hmmm should we maybe take casing into account for this comparison? Otherwise there might be weird side effects if two index where conditions have different case for whatever reason. This might be an issue if the where condition retrieved from the database schema is case converted by the database internally. Then comparing here might fail (not sure).