[2.0] DDC-200 Implemented support for @columnDefinition - even with support to pass the definition to join columns if necessary for relations.

[2.0] DDC-169 - AbstractPlatform::getAlterTableSql() now accepts only a TableDiff instance as argument across all platforms.

[2.0] - DDC-169 - Added DropSchemaSql Visitor - Refactored Visitor package a bit, however its still not very appealing to have that much use statements cluttered in the code

[2.0] - DDC-169 - Began refactoring of DBAL code, introduced object notation for a database schema, including Tables, Indexes, Constraints, Sequences and Columns. Added a CreateSql Visitor which transforms a schema object graph into the required SQL statements to create it. Next: Replacing SchemaTool::getCreateSql() with new syntax...