Support for Python 3.x

I made some small modifications to make `pyprof2calltree` run on Python 3.x.

These are:
- using `list(dct.items())` instead of `dct.items()` in places proposed by the `2to3` script. (Same for `dct.values()`)
- replacing `print >> out_file, 'string` with `out_file.write('string\n')`
- replacing `print 'string'` with `sys.stdout.write('string\n')` (as suggested in the [Python 3 Porting Guide](http://docs.pythonsprints.com/python3_porting/py-porting.html#printing))
- replacing `file(fn)` with `open(fn)`
- replacing `file(fn, mode)` with `open(fn, mode)`
- replacing `isinstance(foo, basestring)` with a call to a helper function `is_basestring(foo)`:

  ```python
  def is_basestring(s):
      try:
          u = unicode
          # Python 2.x
          return isinstance(s, basestring)
      except NameError:
          # Python 3.x
          return isinstance(s, (str, bytes))
  ```

With these changes, I was able to install and use `pyprof2calltree` on Python 3.3 (by means of using the `@profile` decorator from [`profilestats`](https://github.com/hannosch/profilestats) on a simple function). I checked that it (still) works on all the Python versions declared in the trove classifiers, which are:
- `Programming Language :: Python :: 2.5`
- `Programming Language :: Python :: 2.6`
- `Programming Language :: Python :: 2.7`
- `Programming Language :: Python :: 3.2`
- `Programming Language :: Python :: 3.3`