Includes and new declaration function for the Python module

This is a pre-release version. It contains the following improvements:

• allows include of external files from markdown files, with the {{ % include ... % }} directive
• in Python module, introduction of the define_env() function.
  This new function uses a single env argument (the decorator @macro is replaced by @env.macro.
• it is now possible to create jinja2 filters (@env.filter decorator)
• it is possible to access to the whole information in the YAML config file, through the env.conf object.
• the declarare_variables() is still available though it will become deprecated.
• enhanced documentation
• enhanced trace for buildand servecommands