Add moment.relative

No tests for this yet, but just wanna open this up to see if there's interest in merging. If there is, I'll add some tests/docs/cleanup.

We need the ability to parse a date relative to another date. For example, given the input "3:33 PM", I want to be able to parse this value relative to two days from now, instead of the current date/time. This allows you to do the following:

```javascript
moment.relative(moment("2014-03-29"), "3:33 PM", "h:mm A")
```