Encode the output to UTF-8 before writing to file

Having run pyScss with `--watch`, it died with:
```
  File "/usr/local/lib/python2.7/dist-packages/scss/tool.py", line 254, in compile
    dest_file.write(self.css.compile(scss_file=src_path))
UnicodeEncodeError: 'ascii' codec can't encode character u'\uf0d7' in position 816: ordinal not in range(128)
```
Encoding to UTF-8, assuming that's what everyone wants, before writing the file, solves it.

scss/tool.py

@@ -251,7 +251,7 @@ def compile(self, src_path):
 
             print("Compiling %s => %s" % (src_path, dest_path))
             dest_file = open(dest_path, 'w')
-            dest_file.write(self.css.compile(scss_file=src_path))
+            dest_file.write(self.css.compile(scss_file=src_path).encode('utf-8'))
 
         def on_moved(self, event):
             super(ScssEventHandler, self).on_moved(event)