Added explicit implementation of `AbstractType::getName()`

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

#### Proposal
Provide a default implementation of `AbstractType::getName()` which just uses the class name as form name.

#### Caveats
This patch does not prevent name clashes - but you can override the method in your own `AbstractType` implementation. 

Another implementation could be to just use the full class name (and replace `\` with `_` for example - but this would produce really long form names).

#### Problems
Not unit-tested. The current implementation is not directly testable in a unit test - I could move the logic of the class name retrieval in a separate utils class, but I wasn't sure in which class to move it to. `FormUtil` sounds like a generic utils helper, but this logic is not exactly form-related..

PS: meant to be a contribution to the [DX](http://symfony.com/blog/making-the-symfony-experience-exceptional) initiative.