Added 'dayofmonth' Mysql function, updated readme and added section for usage with Silex

Added the following:

* custom function under ```src\Oro\ORM\Query\AST\Platform\Functions\Mysql``` named dayofmonth: just parsing [```DAYOFMONTH```](http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_dayofmonth) Mysql function.
* updated readme with the new function
* added section on readme for usage with Silex for two different service provider:
    * [palmasev/DoctrineORMServiceProvider](https://github.com/palmasev/DoctrineORMServiceProvider)
    * [dflydev/dflydev-doctrine-orm-service-provider](https://github.com/dflydev/dflydev-doctrine-orm-service-provider)

* `DAYOFMONTH(expr)` - is an alias for DAY(expr)