MySQL "ADD COLUMN ... AFTER ..." support

[nanawel]

ZF1 supports MySQL syntax allowing to add a column AFTER another one in an ALTER TABLE statement. ZF2 does not. This PR is intended to fix that.

[alextech]

I want to say "needs a test" but looking at the decorator, there are no existing tests for it where you could have added it. Yet it must be done :) Can you please create an AlterTableDecoratorTest in the same place and same way CreateTableDecoratorTest and add your option in there? (I could assist if unsure how)

[nanawel]

Sure, I'll add it to the PR.

Edit: seems like I included more commits than intended, I'll check that first.

[alextech]

Thank you :)

Edit: I am working under assumption that even though overly database specific modifications are usually rejected, since there is already a MySqlAlterTableDecorator, adding to it should not be a problem.

[nanawel]

Yes, this is my assumption too.

[nanawel]

New test class included, no error detected on PHPUnit on my instance.

[ezimuel]

@nanawel thanks for your contribution!