Increased weight limit (from 200 kg to 500 kg). Fixes issue #21

Also changed implementation of exercise/image download from wger: if a downloaded exercise refers to an image that already exists, the exercise is considered to be duplicate.
Its download will be skipped.
Merge pull request #26 from chaosbastler/wger

Increased weight limit (from 200 kg to 500 kg). Fixes issue #21