cleanup, refactor and document ForeignKeyConstraint asset
add abstraction class for asset identifiers
extend interface for constraints to implement a method for returning quoted column names
adjust ForeignKeyContraint asset to be able to return quoted column names
adjust Index asset to be able to return quoted column names
adjust platforms to make use of quoted index and constraint column names
extend platform tests to test quoted column names in index and constraint generation
mention contraint interface signature changes in UPGRADE instructions for 2.4
fix columns initialization in ForeignKeyConstraint
add phpDoc for new method in constraint interface
add EOL to Identifier class
use strict comparison in Index::spansColumns
fix CS
Merge pull request #302 from deeky666/fix-asset-identfier-quotation

[DBAL-374] Fix asset identfier quotation