Update to YAML 4.1.2

As an added benefit, this makes the ugly 'select_lists' unecessary

--HG--
rename : wger/manager/static/css/yaml401/custom-yaml.css => wger/manager/static/css/custom-yaml.css
Make adding new exercise more accessible to users

* Muscles are no longer a required field
* Muscle names also show their translation, to ease recognition
* Better ordering of form fields

This fixes #63
Replace all occurences of render_to_response() with render()

This is cleaner since there is no need to create a context with
RequestContext.
User 'content_type' parameter instead of mimetype, this is deprecated