fix pipe support on windows

STDIO streams are opened in text mode by default on Windows. This patch
changes the stdin/stdout streams to be in binary mode if they are being
used for I/O from the vpxenc or vpxdec tools.

Fixes issue #216. Thanks to mw AT hesotech.de for the fix.

Change-Id: I34525b3ce2a4a031d5a48d36df4667589372225b