Add `RSVP.sequence` for executing sequence of promises in order

Hi,

Sometimes I need to execute a list of promises in an specific order. `RSVP.all` invokes all the promises at the same time, and this can be a problem depending on the async operation performed. For example, with IndexedDB, executing thousands of write operations at the same time is really bad for performance.

This PR adds a `RSVP.sequence` method to execute a sequence of promises in order. It is undocumented and untested but I wanted to know if you would be interested in something like this first. It is based on [this proposal](http://stackoverflow.com/questions/20100245/how-can-i-execute-array-of-promises-in-sequential-order). I think it's an addition that makes sense for RSVP.

If you are interested I can add some tests and documentation for the method.