Issue #300: Non-ASCII characters in passwords cause error 500 (conservancy/kallithea)

[cl0ne]

New issue 300: Non-ASCII characters in passwords cause error 500
https://bitbucket.org/conservancy/kallithea/issues/300/non-ascii-characters-in-passwords-cause

cl0ne:

**Kallithea version:** 0.3.3

**Steps to reproduce:**

1. Go to log in form
2. Use existing username
3. Fill password field with any non-ASCII characters

**Result:**

"500 Internal Server Error" page shown

Server logs contain error:
```
WebApp Error: <type 'exceptions.UnicodeEncodeError'>: 'ascii' codec can't
encode character u'\u042f' in position 0: ordinal not in range(128)
```

**Expected result:**

On Unicode input validation shows message for invalid password and form accepts valid password