Added tests for Jinja2.

[AndreasBackx]

This is a follow-up on #58. Tests have been added for Jinja2. Following is a checklist of what is or has yet to be done before this PR can be merged.

• Added basic sass_src test for Jinja2.
• Add test for Jinja2 rises TemplateSyntaxError if a variable is passed #54
• Add tests for management command that uses the Jinja2 engine. This will then fix compilescss does not support jinja2 #49.

[AndreasBackx]

Note my commit message on the latest commit:

Added tests for compilescss management command.

* Resolves #49.
* Fixes failing Python 2.7 Jinja2 tests.
* Multiple engines are now supported in the compilescss command.
* Added more info to README regarding Jinja2 compilescss support.

I've added support for using multiple engines in compilescss as well:

manage.py compilescss --engine django --engine jinja2

---

I stumbled upon BaseCommand.requires_system_checks that we could set to False so the command can still without needing to run the system checks. Maybe we could look into this? django-compressor uses it as well.

Shouldn't we add the license from django-compressor because we use parts of their management command's code? They also do it because they have some code from django-compress, see their LICENSE file. While we're at it, can we rename the LICENSE-MIT file to LICENSE?

[AndreasBackx]

My code formatter did this, I can fix this in a follow-up commit along with the other odd ones to make them look less odd after a review. Perhaps to something along these lines:

self.sass_output_style = getattr(
    settings,
    'SASS_OUTPUT_STYLE',
    'nested' if settings.DEBUG else 'compressed'
)

[AndreasBackx]

I apply isort to all of my files, maybe we should start doing this everywhere and add a check for it in Travis along with flake8?

[jrief]

I'll check another project and use its isort config

[AndreasBackx]

Currently it isn't very intuitive to test multiple versions locally. Could we revamp the tox.ini to make this easier? I wrote one for django-safedelete as well.

[jrief]

Thank you so much!