Accents in notebook names and in command-line (nbconvert)

The Problem
-----------

Current stable version had some issues when you try to use accents in the notebook names or in the command line. I had problems with:
 * Use accents in the notebook name
 * Use command-line arguments with accents. In my case I tried to use SphinxTransformer.author = my_name for setting the author in latex documents and my_name included accents.
 * Convert to pdf a notebook with accents in its name.

The two first issues are solved in the development version but the last one is still there.

The proposed solution
---------------------

I added new test cases in the nbconvert test folder for testing those issues. These tests will be useful for maintaining the accents support.

I fixed the issue with the pdf postprocessor with the accents in file names. There were two problems: A mix between str and unicode characters and also an encoding problem in windows (arguments are received in cp437 from the dos console but we must use cp1252 for calling pdflatex subprocess). I saw some code that wrap subprocess.Popen calls in the IPython.utils so I guess that this fix could be implement in a better way...