Python3 compatibility

Update the StringIO import in symposion.sponsorship.views

Massively upgrade symposion

* Remove markitup (to be replaced with Ace editor)
* Use DUA decorators
* Removed custom signup bits
* Upgraded dependencies
* Added migrations
* Namespaced template locations
* Removed html5parser/sanitizer (for now) - parsing functionality
should be moved out entirely to a hooks
* Replaced ProposalScoreExpression object with a function that returns
F() expressions

i18n all part

- forms for labels and helps
- views for success or warning messages
- apps and models verbose_names for admin

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

python3 compatibility

- Things are suggested in python3 porting guide.
https://docs.djangoproject.com/en/1.8/topics/python3/

1. adding ```from django.utils.encoding import
python_2_unicode_compatible```

2. ``` __str__``` instead of ```__unicode__```
https://docs.djangoproject.com/en/1.8/topics/python3/#str-and-unicode-methods

3. Adding ```from __future__ import unicode_literals``` at the top
of your Python modules
https://docs.djangoproject.com/en/1.8/topics/python3/#unicode-literals

4. Removing the `u` prefix before unicode strings;
https://docs.djangoproject.com/en/1.8/topics/python3/#unicode-literals

- also closed #66

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

Enhanced sponsorship features

It is a part of feedback from PyConJP development.
Here picks a sponsorship benefit management enhancement.

* sponsor zip download
* export sponsor data as csv

```
commit eb3261c12c910ec562e016f10431cc48747baef8
Author: Dan Poirier <dpoirier@caktusgroup.com>
Date: Wed Aug 21 11:51:20 2013 -0400

Enhanced sponsor admin page

For #67:

* admin list sorted by name
* not limited to 100 per page
* a sortable visual indicator for each sponsorship benefit
(completed, missing, not applicable)
* also the sponsorship level, admin contact name, and an active
or not indication
* the ones we have today: print logo, web logo, print
description, web description and the ad.
* an action pick list like “email” and check mark the sponsors
* I want to email based on the assets that are missing.
* in subject and body, replace %%NAME%% by sponsor name
```

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

Redirect to dashboard if there are no benefits to edit.

The sponsor object has been created, and we only need to collect details
about the potential-sponsor's benefits. If there are no benefits, the user
should be redirected to the dashboard rather than forced to submit what
appears to be the same form twice.