[DBAL-56] Add portability wrapping layer porting MDB2 functionality to fix case, rtrim strings and convert empty strings to null.

[DBAL-45] Add reserved keywords feature through platforms and as a validator for any Schema instance with subelements. Add Command to check for reserved keywords in current database.

DBAL-88 - Change index retrieval of MySQL to use information_schema.statistics which avoids errors when using reserved words for table names (for example in database driver).

DBAL-84 - Fix VARCHARs being silently set to 255 chars when larger than Maxlength. Now they are converted to Text/CLOB fields automatically. Refactored the VARCHAR code considerably.

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

This reverts commit 3b79401b to allow a sane merge of Juokaz patch.

added supportsReleaseSavepoints(), disabled releasing for Oracle and DB2 (already disabled for MSSQL