[DBAL-42] Optimized column comment change detection. Ignore from columns with a null comment which indicates not set compared to an empty string which says delete comment.

[DBAL-42] Add support to reverse engineer Comments from MySQL, PostgreSQL and ORacle into Doctrine\DBAL\Schema\Column instances and added comment comparison into Comparator.

DBAL-91 - Fix auto-generation of asset names (foreign keys and indexes) which could lead to exceptions due to auto-quoting of PostgreSQL for example. Now hashing the names with crc32+dechex

Fixed oracle integer/boolean type detection in OracleSchemaManager::_getPortableTableColumnDefinition()

DBAL-62 - There is a diference between maximum length (now 4000, oracle is the smallest possible denominator here) of varchars and the default length (255).