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.
I actually get the following errors:

* ``getIndexDeclarationSQL()``
    * Doctrine\DBAL\DBALException: Operation 'Doctrine\DBAL\Platforms\SQLAnywherePlatform::getIndexDeclarationSQL' is not supported by platform.``
* ``getIndexDeclarationSQL()``
    * InvalidArgumentException: Can only create unique constraint declarations, no common index declarations with getUniqueConstraintDeclarationSQL().
* ``getCreateIndexSQL()``
    * passed !

So it seems, there is only ``getCreateIndexSQL()`` which could be tested. And since this test  was entirely written by me and ``getCreateIndexSQL()`` is already tested elsewhere, I don't think we "lose" coverage, nor that the effort would be worth it. What do you think ?