fixed bug when passing unicode queries

This code:
```
# -*- coding: utf-8 -*-
from scholar import ScholarQuerier, SearchScholarQuery

querier = ScholarQuerier()
query = SearchScholarQuery()
query.set_phrase(u'Computer Vision – ACCV 2006')
querier.send_query(query)

print querier.articles
```
produces:
```
Traceback (most recent call last):
  File "scholar_unicode_bug.py", line 7, in <module>
    querier.send_query(query)
  File "scholar.py", line 810, in send_query
    html = self._get_http_response(url=query.get_url(),
  File "scholar.py", line 654, in get_url
    urlargs[key] = quote(str(val))
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2013' in position 16: ordinal not in range(128)
```

The pull request solves the issue.