added get_weight method

The variant could be the physical product while the product a meta container, saleor should not know about this.
Update variants.py

the product variant should be in charge to give the weight, saleor should not know if the ```PhysicalProduct``` is the ```ProductVariant``` or the ```Product```
Update __init__.py

According to the previous commit the delivery should get the weight from the variant or from the product I think
Update variants.py

The ```get_weight``` method should be applied to a ```PhysicalProduct``` since it could have no variants in some forks
Merge pull request #155 from zauddelig/master

A nicer method to give the product weight