missing comma syntax error

A very common mistake when editing json files, is to forget to add a comma, or to add an extra one.

vim-json already detets extra commas, but it does not detect missing ones.

Here is a minimal example of an invalid json file which is not highlighted for errors by vim-json:

``` javascript
{
  "object1": "value1"
  "object2": "value2"
}
```
