Fix persistence exception on a table with a schema on a platform without schema support

This PR solves two related issues with the use of a database schema on platforms (such as SQLite) that don't support schemas.

I discovered the issues when I generated the schema from my Doctrine entities on SQLite (for unit test purposes of my application) whereas my main application uses PostgreSQL.

This is one of my first PR on Doctrine, so sorry if I made some things in the wrong way and I'm open to discussion.

**First problem: table names dots are not converted in the ORM**

On a platform like SQLite, DBAL converts table names with dots (ie. a schema is declared) to double underscores.
However, the ORM doesn't do it, and persisting leads to an exception.

Example:

```
MyNamespace\Mytable:
    type: entity
    table: myschema.mytable
    # ...
```

And then somewhere in the code:

```
$myTable = new MyNamespace\Mytable();
$entityManager->persist($myTable);
$entityManager->flush();
```

This doesn't work in the current version of Doctrine. The table is created as `myschema__mytable` but entities are unsuccessfully saved in `myschema.mytable`.

**Second problem: table names with reserved keywords in a database schema are not correctly escaped**

When a table name is declared as `myschema.order` (or any other reserved keyword), only the reserved keyword part is escaped when creating the table, leading to the creation of a table name like myschema__\`order\`, which is invalid and therefore fails.

**How this PR solves the problem**

The classmetadata now stores in 2 separated properties the name of the table and the name of the schema. The schema property was partially implemented but I now make a full use of it.

When metadata is read (from Annotations, YAML, ...), if the table name has a dot (`myschema.mytable`), it's splitted into 2 parts, and `myschema` is saved in the `schema` table property, and `mytable` is saved in the `name` table property, instead of storing the whole `myschema.mytable` in the `name` table property.

This allows to do specific things about schemas everywhere in Doctrine, and not splitting again parts everywhere it's needed.

By the way, the `schema` property can now fully be used.

For instance, these 2 YAML configurations are valid and do the same thing:

```
MyNamespace\Mytable:
    type: entity
    table: myschema.mytable
```

and:

```
MyNamespace\Mytable:
    type: entity
    table: mytable
    schema: myschema
```

This was something which was not finished to be implented since Doctrine 2.0.

The Default quote strategy now converts back the schema and table names to a unique table name, depending on the platform (e.g. `myschema.mytable` if the platform supports schemas, and `myschema__mytable` otherwise).

As a result, there is no problem anymore and entities can be persisted without getting any exception.
I added some unit tests for this (the same unit tests failed before of course).

There's however a slight tradeoff on performance, as the `getTableName` of the `DefaultQuoteStrategy` class adds some tests to return the correct table name.

This solved these Doctrine issues: [DDC-2825](http://www.doctrine-project.org/jira/browse/DDC-2825) and [DDC-2636](http://www.doctrine-project.org/jira/browse/DDC-2636).

Again, this is one of my first PRs on Doctrine, so if there's anything wrong or if you have any question, feel free to comment this PR.

@Ocramius This PR is about what we talked about in [DDC-2825](http://www.doctrine-project.org/jira/browse/DDC-2825)