Updated uservoice service tracking code.

Now all custom options are in `USERVOICE_WIDGET_OPTIONS`. This could be overriden per-view
with a `uservoice_widget_option` context variable.

For example, to override the default trigger position you could set:

	USERVOICE_WIDGET_OPTIONS = (('addTrigger',
					{"trigger_position": "bottom-left",
                                         "mode": "contact"}),)

`USERVOICE_WIDGET_OPTIONS` accepts any Uservoice's JS SDK configuration
See https://developer.uservoice.com/docs/widgets/overview/
update docs
flatten confiuration. update doc
trigger custom
doc typo
updating tests
update tests
empty key returns no code
assertIn fallback for py2.6/dj1.2
Merge branch 'uservoice' of github.com:mgaitan/django-analytical into mgaitan-uservoice
Add Martín Gaitán to authors list