Add throttle/debounce to quiver.streams

See:

http://lodash.com/docs#throttle
http://lodash.com/docs#debounce

In dart these translate to Stream transformations:

``` dart
Stream throttle(Stream stream, Duration wait, {bool leading: true, bool trailing: true});
Stream debounce(Stream stream, Duration wait, {Duration maxWait, bool leading: false, bool trailing: true});
```

Here is the existing dart core feature request:

http://dartbug.com/8492

Start by locating the quiver.streams library and review the requested throttle and debounce signatures in this issue. Compare their intended behavior with the linked lodash documentation and Dart core feature request; done means both stream transformations support the stated options and wait durations.

Written by the indexing model from the issue text.