pyScss fails on UTF-8 data

I checked using the ruby version, here's what it output:

```
$ sass /tmp/quotes.scss 
@charset "UTF-8";
q {
  quotes: "“" "”" "‘" "’"; }
```

The error that comes up is:

```
E             File "/XXX/pyScss/scss/expression.py", line 56, in do_glob_math
E               cont = str(cont)
E           UnicodeEncodeError: 'ascii' codec can't encode character u'\u201c' in position 1: ordinal not in range(128)
```

I could look into getting this test to pass, but right now I just don't have time.  I thought I'd open the pull request with just a failing test in case somebody else had time right now to look into it.