[DBAL-374] Fix asset identfier quotation

This PR introduces a real solution for the issues discussed in [DBAL-374](http://www.doctrine-project.org/jira/browse/DBAL-374).
It adds the possibility to retrieve quoted column names from <code>Index</code> and <code>ForeignKeyConstraint</code> assets if it is necessary (e.g. column name is a keyword reserved by the platform).
In general column names are wrapped into an abstraction class <code>Identifier</code> which inherits from <code>AbstractAsset</code> inside <code>Index</code> and <code>ForeignKeyConstraint</code>. This allows to get a quoted representation of each column name via the <code>AbstractAsset::getQuotedName</code>.
All platforms are updated to make use of the new functionality.