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 see what I mean, so uppercasing everything would be a straighforward solution. However what if the where condition include some literal string ? it should probably not be uppercased. So instead of doing something flaky (I don't know how to uppercase only non-literal strings), I would rather chose to do nothing a tall and leave the responsibility to the developers to use whatever case is working for them.

What do you think ?