Length of fixed string type (char) is ignored on Postgre schema update

This PR should fix an issue for the fixed string type columns using PostgreSQL.

I've created a basic Symfony 2.6. Application and using a PostgreSQL 9.3. database. In this dummy project, I've created the following Entity:

``` php
<?php

namespace Acme\DemoBundle\Entity;

use Doctrine\ORM\Mapping AS ORM;

/**
 * @ORM\Entity
 * @ORM\Table(name="test")
 */
class Test
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer", name="id")
     * @ORM\GeneratedValue
     */
    public $id;

    /**
     * @ORM\Column(type="string", name="name", length=2, options={"fixed" = true})
     */
    public $name;
}
```

The table is successfully created after a schema update, but when another schema update is executed without any changes to the entity, an ALTER query is executed:

``` sql
ALTER TABLE test ALTER name TYPE CHAR(2);
```

After I searched in the code of doctrine dbal, I discovered that the length of the char column is not fetched correctly in `PostgreSqlSchemaManager::_getPortableTableColumnDefinition()`. In my example, length results as `null` and will be later set to 255 in the `Comparator`, which isn't the correct length from the database column. The comparation of the schemas results in a change of char-length.

For simplicity, I extended the if-condition in `PostgreSqlSchemaManager` to:

``` php
if (strtolower($tableColumn['type']) === 'varchar' || strtolower($tableColumn['type']) === 'bpchar') {
```

The fix I've contributed does help to parse the length in my case, but when I look into `PostgreSqlPlatform::initializeDoctrineTypeMappings()` there could be other types with the same problem, that are mapped to the doctrine string type.

``` php
// ... part of PostgreSqlPlatform::initializeDoctrineTypeMappings()
            'varchar'       => 'string',
            'interval'      => 'string',
            '_varchar'      => 'string',
            'char'          => 'string',
            'bpchar'        => 'string',
            'inet'          => 'string',
```

I guess we need to cover `char` and the others too.

`PostgreSqlSchemaManagerTest` also doesn't cover existing code. How is this going to be tested? Does the SchemaManagers need some tests in general?