Image placeholder unicode issue

Here's bugfix for the issue I caught trying to upload image file contained cyrillic symbols.
```
  File "/srv/www/test/ENV/local/lib/python2.7/site-packages/pages/placeholders.py", line 285, in save
    filename = get_filename(page, self, data)

  File "/srv/www/test/ENV/local/lib/python2.7/site-packages/pages/placeholders.py", line 250, in get_filename
    placeholder.name + '-' + str(time.time()) + '-' + str(data)

UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)
```