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.
we shouldn't introduce more protected properties IMO. Could you please make it private? Those protected one are only there for BC. If there will be the need to access the options in the future, we can still provide a getter or something...