minor fix to make piio.read/write compatible with unicode strings

example:

    piio.read(u'/tmp/lena.png')

now works, as well as:

    piio.read('/tmp/lena.png')

You don't need to type:

    piio.read(str(u'/tmp/lena.png'))
mkstemp now appears only once
change piio's iio
Merge branch 'master' of https://github.com/gfacciol/iio