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.
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.
Adapt formatting to pre-existing content
Surround ! with spaces according to code style
Merge pull request #1027 from PowerKiKi/feature-partial-indexes

Support for Partial Indexes for PostgreSql and Sqlite