Crashes on filenames with non-english (non-ascii?) characters

[moezzie]

Great work on writing glue. Love it!

Noticed that running glue on files containing non-english characters like ö, å, ä produces a crash on OS X 10.10.4.
Included the crash log below

Version: 0.11.0
Python: 2.7.9 (default, Jan 29 2015, 06:27:40)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)]
PIL version: 1.1.7
Platform: Darwin-14.4.0-x86_64-i386-64bit
Config: {'css_dir': '/Users/x/portraits/small_sprite', 'less_dir': False, 'force': False, 'json_dir': False, 'crop': False, 'less_template': None, 'css_url': u'', 'global_template': None, 'css_namespace': u'sprite', 'html_dir': False, 'css_cachebuster': False, 'recursive': False, 'scss_template': None, 'css_pseudo_class_separator': u'__', 'enabled_formats': ['img', 'css'], 'generate_css': True, 'source': '/Users/x/portraits/small', 'optipng': False, 'follow_links': False, 'imagemagick': False, 'each_template': None, 'json_format': u'array', 'watch': False, 'css_sprite_namespace': u'{sprite_name}', 'padding': u'0', 'ratio_template': None, 'ratios': u'1', 'caat_dir': False, 'optipngpath': None, 'css_cachebuster_only_sprites': False, 'scss_dir': False, 'generate_image': True, 'cocos2d_dir': False, 'imagemagickpath': None, 'algorithm': u'square', 'css_cachebuster_filename': False, 'img_dir': '/Users/x/portraits/small_sprite', 'png8': False, 'quiet': False, 'project': False, 'css_separator': u'-', 'algorithm_ordering': u'maxside', 'debug': False, 'output': '/Users/x/portraits/small_sprite', 'margin': u'0', 'css_template': None, 'ignore_filename_paddings': False}
Args: ['/usr/local/bin/glue', 'small', 'small_sprite']

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/glue/bin.py", line 235, in main
    manager.process()
  File "/usr/local/lib/python2.7/site-packages/glue/managers/base.py", line 14, in process
    self.save()
  File "/usr/local/lib/python2.7/site-packages/glue/managers/base.py", line 42, in save
    format.validate()
  File "/usr/local/lib/python2.7/site-packages/glue/formats/css.py", line 142, in validate
    class_names = [':'.join(self.generate_css_name(i.filename)) for i in self.sprite.images]
  File "/usr/local/lib/python2.7/site-packages/glue/formats/css.py", line 209, in generate_css_name
    if css_pseudo_class_separator in filename:
UnicodeDecodeError: 'ascii' codec can't decode byte 0xcc in position 6: ordinal not in range(128)