Support sorting parameters

## Feature
On listing APIs (items, users, suspensions, later stdlib - see #15), support a `sort` parameter to control sorting of the output.

## Details
* The parameter contains multiple sorting criteria (with descending precedence), separated by `+`.
* Descending sorting can be specified by prefixing the criteria with an exclamation mark `!`.
* The allowed sorting fields can differ from DB names, and must explicitly be documented.
* Unknown sorting fields: throw an error or ignore silently?

##  Example:
```
/items/list?sort=name+!version
```

## Merging
According to the current release plan, this will go into `0.2.0`. Most importantly, #15 must be merged first into master (and master into this branch), as it must be worked on.