Improve Comparison::CONTAINS: allow to use the SQL wildcard characters


This pull additional to doctrine/doctrine2#1150
And allow to use the [SQL wildcard characters](http://www.w3schools.com/sql/sql_wildcards.asp) `%` and `_`  for  `Comparison::CONTAINS`

Allow to build the `contains` expressions like:
```php
Criteria::expr()->contains('myField', 'some string%');
Criteria::expr()->contains('myField', '%some string');
Criteria::expr()->contains('myField', '%10\%'); // search for 10% at end of the string
Criteria::expr()->contains('myField', 's_m_ string');
```

I think, this changes can make b/c issue for existing `Comparison::CONTAINS` expressions, when someone use not escaped `%`, `_` .