Imported print function for Python 2 compatibility

Subsample gives an error on Python >=2.6:

    $ subsample foo.txt --approximate -f 0.1
    ...
      File "/Users/anto/Documents/src/subsample/subsample/main.py", line 61
        'Use another algorithm or save to a file first.'), file=stderr)
                                                               ^
    SyntaxError: invalid syntax

This pull requests corrects the error by importing the print function from `__future__`.