Add another 'unit test' for the interpreter.

This unit test shows that after a given number of run
of the same file, the error changes.
Unfortunately, this can not be automatically verified, since
the result is printed in the python error stream, and cannot be
redirected.
Correct the PyRun_File use. Close the file once finished. Solves error pointed in previous commit.
Avoid memory leak.
Add a method allowing to execute a file and retrived the error (if any) via a string.
Add a working python file to test the interpreter.
Merge pull request #10 from francois-keith/master_2

Improve the interpreter
Fix the problem related to the coexistence of several python threads, reference: https://docs.python.org/2/c-api/init.html#thread-state-and-the-global-interpreter-lock

Conflicts:

	src/interpreter.cc
Merge pull request #1 from graiola/upstream

Update with stack-of-tasks fork