[WIP] Configuration system refactor

See issue #285.

### Basic ideas ###
- Reorganize the configuration properties in a more rational hierarchy
- Support for multiple formats
- Validate configuration

### Implementation details ###

Some of the following points should be discussed.

- Based on [Symfony Config Component](http://symfony.com/doc/current/components/config/index.html)
- Support for `.php, .ini, .json, .yaml, .xml` formats.
    - Ini file loader accepts both `.ini` and `.properties` files. 
    - Inside ini file, we can define a hierarchy in standard way (with sections and array definitions) or in a "nested" way, inspired by [Zend Framenwork Config](http://framework.zend.com/manual/2.2/en/modules/zend.config.reader.html).
- Ability to define some parameters, in any configuration format, delimited by `%` (see `Symfony\Component\DependencyInjection\ParameterBag` class).
Example:

``` yaml
# yaml
database:
    adapter: mysql
    host: localhost
    dsn: %adapter%:host=%host%
```  
becomes:

```php
// php
array(
    'database' => array(
        'adapter' => 'mysql',
        'host' => 'localhost',
        'dsn' => 'mysql:host=localhost'
    )   
);
```
- I've put all new classes in `Propel\Common\Config` namespace. I've choosen `Propel\Common` because, theoretically, configuration system can be used both at generation and runtime, even if, for the most part, is used at generation time.
- I supposed to define a single configuration file, named ~~propel-config.ini~~  `propel.ini` (or .yaml or some other supported format) which should be located in project root directory or in a sub-directory named "conf" or "config" (we can choose something else). This file also replace `runtime.conf` and `buildtime.conf`.
- ~~Default configuration, shipped with Propel, is placed in `tools/Generator/default-config.yaml` file (may be it could be better `tools/Config/default-config.yaml`). Default properties can be override by project configuration, which can be override by options passed to command line.~~ Moved to step 2 

### Roadmap ###

- [x] Step 1: add new classes and relative tests
- [x] Step 2: ~~move all default options from command classes to `default-conf.yaml` and~~ implement `Propel\Common\Config\PropelConfiguration` class to validate configuration and load defaults, via TreeBuilder. Remove all not more used options and define a better property structure.
- [x] Step 3: refactor all those classes using configuration.
- [x] Step 4: update documentation

Each step can be merged separately.