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.
Fix more code style as mentionned in PR comments
Surround ! with spaces according to code style
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.
Merge pull request #600 from PowerKiKi/feature-partial-indexes

Support for Partial Indexes for PostgreSql and Sqlite