Widget URL parameters

Overview
--------------

- Some suggested linter fixes / minor refactoring to `views.py` and `widgets.py`.
- Added parameterization to the registry. 

Example
------------
```
router.register(views.GitHubIssuesWidget,
                'github-issues',
                github_user="[A-Za-z0-9_-]+",
                repository="[A-Za-z0-9_-]+")
router.register(views.GitHubPullRequestsWidget,
                'total-prs')

urlpatterns = patterns(
    '',
    url(r'^legacy/github-issues/(?P<github_user>[A-Za-z0-9_-]+)/(?P<repository>[A-Za-z0-9_-]+)/?', views.GitHubIssuesWidget.as_view(), name='github-issues'),
    url(r'^legacy/total-prs/?', views.GitHubPullRequestsWidget.as_view(), name='total-prs'),
) + widget_router.urls

```

Resulting in:
```
^dashboard/ ^legacy/github-issues/(?P<github_user>[A-Za-z0-9_-]+)/(?P<repository>[A-Za-z0-9_-]+)/? [name='github-issues']
^dashboard/ ^legacy/total-prs/? [name='total-prs']
^dashboard/ ^$ [name='dashboard']
^dashboard/ ^widgets/github-issues/(P<github_user>[A-Za-z0-9_-]+)/(P<repository>[A-Za-z0-9_-]+) [name='widget_github-issues']
^dashboard/ ^widgets/total-prs/ [name='widget_total-prs']
```