BUG: Fix str/unicode warnings from SQLAlchemy on python 2.

When running with `reset_db=True` on python 2, several SQLAlchemy operations
were performed with bytes where unicode was expected, resulting in
warnings like the following.

```
/home/ssanderson/.virtualenvs/jupyterhub/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py:573:
SAWarning: Unicode type received non-unicodebind param value.
  param.append(processors[key](compiled_params[key]))
```

Fixes a few stray non-unicode literals and adds a unicode safe wrapper
for `getpass.getuser`.

jupyterhub/utils.py

@@ -4,9 +4,12 @@
 # Distributed under the terms of the Modified BSD License.
 
 import binascii
+import getpass
 import errno
 import os
 import socket
+
+from six import text_type
 from tornado import web, gen, ioloop
 from tornado.httpclient import AsyncHTTPClient, HTTPError
 from tornado.log import app_log
@@ -21,6 +24,14 @@ except NameError:
     class TimeoutError(Exception):
         pass
 
+
+def getuser_unicode():
+    """
+    Call getpass.getuser, ensuring that the output is returned as unicode.
+    """
+    return text_type(getpass.getuser())
+
+
 def random_port():
     """get a single random port"""
     sock = socket.socket()