Add support for specifying the interpreter on the command-line in rpdb2.

Warning: this breaks winpdb.py for the moment, it will be fixed shortly.
- remove some default argument for interpreter

- function to fetch the default interpreter for launching  a program
- make winpdb compatible with specifying its own interpreter on the command-line
- add dialog to winpdb to choose Python interpreter upon launch

- rename WINPDB_WILDCARD to WILDCARD_WINPDB
typo
Add unit-tests for get_python_executable
- can now specify interpreter when launching a program from the console

- add unit-test for console launch command-line parsing
Improve functional tests :

- detect line on which to point breakpoint directly in debugged file
- control the display level
Do not make the interpreter break before exit the default behavior.

- adds an option to specify break before exit on the command-line : --stop-on-exit