StreamBuffer

For those times you need to just read(10) bytes.
Update readme
Buffer limits + fragmenting

Allow the buffer to have an upper limit before pausing the backing stream.
Added test to show piping of multiple streams works.
Cleanup.
More cleanup after a second pair of eyes.
Improvements: optional Underflow and non-lists

* Allow StreamBuffer to underflow if there are readers and the
  stream is done.
* StreamBuffer can accept a Stream<Foo> or a Stream<List<Foo>>,
  read(5) will still return a single, 5 entry list of Foo.
  Good for non-buffered streams.
Merge branch 'master' of https://github.com/google/quiver-dart
Merge remote-tracking branch 'upstream/master'
Reviewer comments
Merge branch 'clockwatcher', remote-tracking branch 'origin'
Add another example to README
Merge commit 'ecfef0ddf10d3c20065b6796eec96a34d9acc24e'; commit 'a8b9c67243b0c4d852ead421dc20e81bee2e5ad4' into clockwatcher