Improved introspection on Symfony Forms

I've added some improvements to form introspection.

Now custom form types can be defined/intropected hooiking into `nelmio_api_doc.parser.form_type_parser` tag.

Currently 4 test will fail (`NelmioApiDocBundle\Tests\Parser\FormTypeParserTest::testParse`).
The previous output was: 
```
Parameter 	Type 	Required 	Format 	Description
form[name][] 	|array of strings 	|? 	|? 	|?
```
The current output is just: 
```
Parameter 	Type 	Required? 	Format 	Description
form[name][] 	|string 	|? 	|? 	|?
```

I've not yet added tests ... :(