Add timezone support when rendering datetimes

[jcass77]

Provides timezone support that is aligned with standard Django.

• introduces APSCHEDULER_DATETIME_FORMAT which can be used to configure the time format that should be used. The default just adds seconds to Django's default DATETIME_FORMAT setting.
• timestamps will be rendered in the local time if timezone support is enabled via USE_TZ = True.
• unit tests for all changes made.

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 0.4%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##           master      #43     +/-   ##
=========================================
+ Coverage   72.29%   72.69%   +0.4%     
=========================================
  Files           9        9             
  Lines         296      304      +8     
=========================================
+ Hits          214      221      +7     
- Misses         82       83      +1

Impacted Files	Coverage Δ
django_apscheduler/admin.py	0% <0%> (ø)	⬆️
django_apscheduler/util.py	100% <100%> (ø)	⬆️

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[jcass77]

The travis build appears to be failing for Django 1.8, and I could not find a trivial solution for making this PR backwards compatible.

Extended support for version 1.8 of Django ended on April 1, 2018 - do we really still need to support it?

I have taken the liberty of pushing 86b7234, which drops support for legacy versions of Django.

[jarekwg]

Thanks!