Print to stderr instead of stdout

As of now, wasabi just calls `print()`, which by default prints to the stdout. In my opinion, this is wrong, as stdout (in the Unix ideology) should be used to output the result of the program (for eventual piping), while stderr should be used for warnings and errors.

I propose adding a "stream" parameter to `Printer.__init__()`, which by default should be set to `sys.stderr`. You can then use it inside the `print()` call.

If you're low on free time, I can implement this, too :)