Merge pull request #17 from hannseman/master

Fixes #11 -- Refactor jinja2 global call_viewlet to jinja2 extension
Merge pull request #18 from hannseman/master

Bump version to 1.2.0
Update Jinja2 integration in README
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