Add feature: sort queryparams can be relationship fileds

[babywolfh]

sort queryparams can be relationship fileds

[coveralls]

Coverage decreased (-0.1%) to 93.52% when pulling 05106a491a18782566f10a77d1554b857383631a on babywolfh:sort_relationships into 268e94808573033e0232007a887982207f715e9a on miLibris:master.

[akira-dev]

I think it is a good start to implement sorting on relationship fields but it would be better that you can specify the related field to sort on instead of only use to related "id" field. ex:

{'field': field, 'order': order, 'relationship_field': related_field}