Improving compatibility towards Python 2 and 3

 * fixed a lot of issues due to inconsistencies of the code initially
    written for Python2, mainly confusions between byte strings
    and unicode strings. The new code tries to use byte strings for
    file-like streams and unicode strings for in-memory objects.
  * added JavaDoc strings to document thoroughly functions and methods,
     mainly in odf/opendocument.py
  * added a config.dox file to allow one to build a documentation in HTML
    and LaTeX, for library developpers and users.
  * added many assert() clauses to ensure the types of parameters in
    functions and methods
  * modified the utilities to make them usable with both Python2 and Python3
  * implemented a feature announced in csv2ods manfiles but not previously
    active: -c / --encoding switch to take in account the encoding of
    the CSV source file.