Merge pull request #19 from lundberg/master

Update README
Load jinja Environment lazily and cache it

This avoids a circular dependency when a project's jinja Environment is declared at the module level and depends on django settings (since django-viewlet previously imported the Environment at project startup).
Merge pull request #20 from heyman/env-dependency-fix

Load jinja Environment lazily and cache it
Bump version to 1.2.1 and pep8
Merge pull request #21 from hannseman/master

Bump version and pep8-fixes
Added test exposing the bug in issue #23
split the Viewlet.call() method
the acctual calling of the viewlet function is now exec. by Viewlet._call() aswell as all the cache handling
and the logic left in call() is only handling the rendering of context viewlets
this to enable refresh() to only trigger whats necessary
Changed Viewlet.refresh() to call self._call() instead of self.call()
Refactored a test affected by the refresh changes
flake8 fixes
version bump
Merge pull request #24 from gardeman/refresh-refectoring

Fixes #23
Use Travis matrix to speed-up testing
Control cache backend via settings and argument
Pep8
Add refresh attribute on viewlet function
Rename backend argument to `using`
Add examples of setting cache backend to readme
Add tests for settings cache backend. 
Fix default timeout in decorator.
Small fixes in tests.
Django 1.3 does not have override_settings