DDL component fixes

* fix some column definitions (blob, text, integer, bigint, varchar, char)
* add column def (datetime)
* add Index constraint type
* add AlterTableDecorator for Platform
* adjust tests to be compatible with new definitions
@Ocramius Thanks for elaboration! The signature remained the same, but the internal logic changed as you pointed out. The issue here is: default value could not be used by Blob data type. That means that whoever used Blob column before either didn't set default or created some fixes to bypass the flaw.
That's why I don't think there could be any BC with bugged feature in the first place.