Fix 'makemessages' django command when jade templates have non-ASCII chars

The `manage.py makemessages` django command was throwing an UnicodeDecodeError exception when there was jade templates with non ASCII chars:

    Traceback (most recent call last):
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/base.py", line 240, in run_from_argv
        self.execute(*args, **options.__dict__)
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/base.py", line 283, in execute
        output = self.handle(*args, **options)
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/base.py", line 413, in handle
        return self.handle_noargs(**options)
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/commands/makemessages.py", line 264, in handle_noargs
        potfile = self.build_pot_file(localedir)
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/commands/makemessages.py", line 292, in build_pot_file
        f.process(self, potfile, self.domain, self.keep_pot)
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/core/management/commands/makemessages.py", line 74, in process
        content = templatize(src_data, orig_file[2:])
      File ".../python-envs/hands-on/lib/python2.7/site-packages/django/utils/translation/__init__.py", line 172, in templatize
        return _trans.templatize(src, origin)
      File ".../p/pyjade/pyjade/ext/django/compiler.py", line 68, in templatize
        html = process(src,compiler=Compiler)
      File ".../p/pyjade/pyjade/utils.py", line 225, in process
        return _compiler.compile().strip()
      File ".../p/pyjade/pyjade/compiler.py", line 83, in compile
        return unicode(u''.join(self.buf))
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 28: ordinal not in range(128)

With the fix, the jade template source is converted to unicode before the processing and now it works.