Clear the loop variable in addition to the callback list.

Closes #844.
Start next release notes
Unify lists of errnos used in IOStream.

We generally want to treat aborted connections the same regardless of the
specific errno, but some calls caught ECONNRESET and ECONNABORTED
while others only caught ECONNRESET.  Reported in #828.
Clear the write buffer when an IOStream is closed.

Closes #828.
Fixed laziness of current_user in UI modules, and added tests for current_user. #820
Fix exception in StaticFileHandler when range requested is larger than file.
Correctly handle EAGAIN when writing to PipeIOStreams

BaseIOStream._handle_write needs to catch IOError and OSError as
well as socket.error in order to handle EAGAIN when writing to a
PipeIOStream (which doesn't appear to raise socket.error when the
buffer is full).

This also provides consistency with the exceptions caught by
BaseIOStream._read_to_buffer.
Merge pull request #848 from SuprDewd/issue_820

Fixed laziness of current_user in UI modules, and added tests for current_user.
Merge pull request #851 from taguchimail/master

Correctly handle EAGAIN when writing to PipeIOStreams
Make poll timeout patchable.
Merge pull request #859 from schlamar/patch-1

Make poll timeout patchable.