Support multi-key merging

[jeffh]

Provide a declarative way to merge object keys into an object. e.g.

{
    "text": "First",
    "date": "2013-12-22",
    "time": "14:23:45"
}

And then do something like (magic code):

HYDMapObject(HYDRootMapper, [Post class],
    @{@"text": @"text",
      HYDAccessKey(@"date", @"time"): HYDMapStringToDateFormatter(HYDMapArrayToString(@"date", @"T"), HYDDateFormatRFC3339)}
);

[jeffh]

This is supported as of e79e5e0, although there's not much assistance in Hydrant after an accessor emits an array right now.