Automatic custom skeleton dirs

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: 
Todo: -
License of the code: MIT
Documentation PR: -

It's working the Symfony's way: it provides default include paths to look into for custom templates.

Instead of getting its skeletons from the vendor's directory, the CRUD generator now look first in the following directories:
`[entity's bundle]/Resources/SensioGeneratorBundle/skeleton/crud`
`app/Resources/SensioGeneratorBundle/skeleton/crud`

Then if no custom views were found, it looks into the vendor's default skeleton dir.