Fixed single quote in comments

This is a little fix on unescaped singlequotes in column comments.

I only tested on Postgresql.
The changes in AbstractPlatform.php and SQLAnywherePlatform.php are not tested but the quote string escaping is pretty standard... i guess.
Can we have a distinct test for this method, too? Because currently we make no assumption about the quote character in the test suite.

```php
protected function getStringLiteralQuoteCharacter()
{
    return "'";
}

public function testGetStringLiteralQuoteCharacter()
{
    $this->assertSame($this->getStringLiteralQuoteCharacter(), $this->_platform->getStringLiteralQuoteCharacter());
}
```
This simple test should protect us against possible regressions.