IOStreams now save the exception object that caused them to close.

Closes #494.
Connection errors can happen here too.
Remove use of string exceptions in gen_test.

These lines are not reached, but if they were they would fail with
a less clear message than intended.

Closes #520.
Another round of minor whitespace cleanups from autopep8
Add future import to this new file for consistency
Remove unused variables
Release note updates
Doc updates
Fix zope.interface/py25 dependencies for other tox.ini files
Quick hack to make this test pass on cygwin
Set version number to 2.3rc1
Restore context for all read methods.

Prior to 2db0aceb32f5c042f5306e72a4679580b4359f34 this was being done
properly, but the refactor removed stack_context.wrap causing some issues
in our upstream context managers.
Merge pull request #521 from yoinc/master

Restore context for all read methods
Final 2.3 release notes
Set version number to 2.3
Timeouts in SimpleAsyncHTTPClient now raise HTTPError

Previously, they ran the callbacks themselves.
Post-release version bump
Set alarms on child processes in process_test.

These alarms used to be there, but got dropped in a previous change
because I had assumed the child processes would inherit the parent's
alarm.
Update README to match tornadoweb.org
Use https for all links to github.

Github redirects everything to https anyway, so the links might as well
start out secure.