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.
Fix code style as mentionned in PR comments
Refactor partial into options array

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.
Remove not needed stuff from the PHPUnit boostrap file
Simplify getSQLForSelect
Simplify getSQLForSelect more
Use a strict check against null
Remove some evil spaces
Make the $alias parameter in the `from` method optional
Make code Ocramius certified
Merge pull request #646 from JeroenDeDauw/noalias

Make the $alias parameter in the `from` method optional
Update docs to relfect the changes to QueryBuilder::from made in #646
Merge pull request #647 from JeroenDeDauw/docs

Update docs to relfect the changes to QueryBuilder::from made in #646
Fix more code style as mentionned in PR comments
Surround ! with spaces according to code style
Update docs to include warning when using object type with PostgreSQL
Merge pull request #653 from exi/documentation-fix-postgresql-object-type

Update docs to include warning when using object type with PostgreSQL
Compete BC notice with addUniqueIndex()
Cover Index options with unit tests
Deactivate partial index support for Sqlite

While technically it could work, the schema introspection of Sqlite
platform has severe flaws preventing us to provide reliable support
of partial (and non-partial) indexes for the moment.