Add array type/array object literal syntax

[lvivier]

@Marak this is a bigger one. This PR extends the array-typed properties in mschema:

{
  var point = {
    "name": {
      "type": "string"
    },
    "coords": {
      "type": "array",      // can use "array" type
      "required": true,     // can be required
      "items": [{           // `items` can be a string, object, or array of strings/objects
        "type": "number",   // if an array, the length has to match and the type(s) of each item
        "min": -90,
        "max": 90
      }, {
        "type": "number",   // each member can have a different type
        "min": -180,        // and different constraints
        "max": 180
      }]
    },
    "tags": {
      "type": "array",
      "minItems": 1,        // can have minimum # of items
      "maxItems": 10,       // and maximum # of items
      "items": {
        "type": "string"    // all items must be of the given type
      }
    }
  };
}

This PR also resolves an issue where an array is accepted as a value for object-typed properties.

[Marak]

The introduction of the items keyword seems like a major change to the way we are handling arrays and may cause some issues.

At first glance it looks like we might be OK to merge, but I'm going to want to pull locally and perform a bit more testing. Will let you know soon.

Thanks!