Simplified doModifyLimitQuery per @deeky666 comments, added test case for orderby with parenthetical expression

Added doModifyLimitQuery for SQLServer2012Platform that uses OFFSET... FETCH instead of the awful SQL Server 2008 window function garbage

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.

Unit tests currently only cover SQLite, MySQL and PostgreSQL.

Tests for other platforms have been marked as incomplete.

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.

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.

with assertEquals