Extend `CustomerWrapper` support for `TokenInterface`

Fixes #347
I'm not confortable with empty PHPDocs either. We need to set a convention to describe tests.

A simple c&p of the function name?
```php
/**
 * Load customer from token
 */
```

An extended description?
```php
/**
 * CustomerWrapper::loadCustomer gets current customer from security token
 */
```

Would make sense something like a behat scenario?
```php
/**
 * Given a security token pointing to a valid customer C
 * When  I ask the wrapper to load a customer
 * Then  it returns C
 */
```

I do not have an opinion about this yet on this.
Shall we discuss this on gitter with the community?