Add apostrophes to xhtml_escape.

Closes #833.
Close pipe file descriptors if subprocess.Popen fails.

Closes #834.
Clear local variables in IOLoop to allow GC before we start polling.

Closes #836.
Use TracebackFuture for all internal Futures for better tracebacks on py2.x.
Update FacebookGraphMixin with new authorization url.

The old url still works, but results in an extra redirect.