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.
Same as above. No `elseif` needed here:

```php
if ($this->hasOption('where') && $other->hasOption('where') && $this->getOption('where') == $other->getOption('where')) {
    return true;
}

if (!$this->hasOption('where') && !$other->hasOption('where')) {
    return true;
}

return false;
```