Customize default Python shell as `py-shell-name'

`py-shell-name' might be an installed executable as
shell command `type' would display, but also a
PATH/TO/(I)PYTHON, of a virtualenv for example

Most python-mode.el commands start with prefix `py-'

`M-x py- TAB'
displays a list of them in completion-buffer. 
See also commands list delivered in directory doc.

List virtualenv related `M-x virtualenv- TAB'
resp. Pymacs commands `M-x pymacs-'  

Commands related to a specific shell start with
it's name as `ipython-complete'.
Open an installed shell by 

M-x SHELL

With prefix C-u user is prompted to specify a PATH-TO-LOCAL-SHELL
Also evaluating

(py-shell nil DEDICATED PATH-TO-LOCAL-SHELL)

if DEDICATED is set to `t', shell will get an unique name.

Install a local shell by evaluating

(defun MY-LOCAL-SHELL ()
  (interactive)
  (py-shell nil DEDICATED PATH-TO-LOCAL-SHELL))