add sort by parameter

[andreformento]

closes #27

cc @giuliocc @jvanz

This simple version allow front to use all sort options

Test

Use the new parameter sort_by for search by:

• [ ] relevance (elasticsearch relevance)
• [ ] date by descending
• [ ] ascending order

And do the same with some city:

• [ ] relevance (elasticsearch relevance)
• [ ] date by descending
• [ ] ascending order