Add QuantityHandler

Similar assumptions as in TimeHandler (see #49):
* ~~The equality field is a simple, human readable string representation.~~
* ~~Ignore lower/upper bounds when searching.~~
* ~~Ignore decimal places beyond PHP's build-in "14 significant digits" string conversion.~~
* ~~StringHasher is used to be able to have a maximum length. Otherwise it would be unlimited since the unit doesn't have a limit.~~
* Order by float value. Ordering by unit doesn't make any sense.
* No unit conversion (for now, may be implemented in a future version).
I always found the `DECIMAL` a.k.a. `NUMERIC` type useless. You need to know the exact number of decimal places left and right of the comma when declaring the column (and yes, this defines how values are stored, it's not for formatting). In almost all use cases I'm aware of we don't know this. We clearly don't know it in our use case.

So it mus be `FLOAT`.

DBAL's `FLOAT` [maps to](http://doctrine-dbal.readthedocs.org/en/latest/reference/types.html#mapping-matrix) MySQL's `DOUBLE` which is what I want and expected.