Add api to get item history

**Depends on #23**
/item//history
Return JSON array where each element is one row of the history.
```
{
'datetime': the datetime of the change,
'old_val': string old value, human readable,
'new_val': string new value, human readable,
'field': human readable name of the field that changed. "Purchase price" not "purchase_price"
}
```