Removed $cmd args/properties (Mongo command character)

Submitting this pull request to avoid raising the `MongoDB command prefix option is no longer used` error when using the latest doctrine/mongodb (which deprecated `$cmd` in doctrine/mongodb pull request #131: https://github.com/doctrine/mongodb/pull/131). 

I did leave three literal `$` artifacts in `__construct()` methods of some extending objects (`\Doctrine\ODM\MongoDB\Query\Query`, `\Doctrine\ODM\MongoDB\Query\Expr` and – even though not strictly required to make code pass unit tests – `\Doctrine\ODM\MongoDB\Query\Builder`), for compatibility with versions 1.0.3 and earlier of doctrine/mongodb.

Resulting code passes all unit tests (aside from `Doctrine\ODM\MongoDB\Tests\Functional\DateTest::testOldDate`, which produced an error before the changes and still does afterwards).