Implemented internal python shell.

Implemented internal python shell.

This new kind of shell is intended to be used for generic
communication related to defined configurations. The main difference
with global or dedicated shells is that these ones are attached to a
configuration, not a buffer. This means that can be used for example
to retrieve the sys.path and other stuff, without messing with user
shells.

New Variables:
    * python-shell-internal-buffer-name,

New functions:
    * python-shell-internal-get-process-name
    * run-python-internal
    * python-shell-internal-get-or-create-process
    * python-shell-internal-send-string (makes python-send-receive obsolete)