Statik

* Further fixing issue 59, where the previous fix (erroneously) caused all DateTime fields
to be compulsory. This version makes them optional again.

* Fixing issue 59 - now using [python-dateutil](https://dateutil.readthedocs.io/en/stable/)
to parse date/time fields that cannot automatically be parsed during data loading.
* Fixing issue 60 - adding new feature to support self-referencing models.

* More minor improvements to the error logging, especially from the database interface.

* Started refactoring the error handling system (in an attempt to help with 56). The error
system is now more appropriately hierarchical, and Statik has more control over how error
messages are displayed. There is, however, still more work to be done to make the
console/logging output more user-friendly.
* [python-colorlog](https://github.com/borntyping/python-colorlog) has now also been included
to aid in differentiating between error and informational messages (especially during
watching of project changes).

* Fixed 49 - the `httpwatcher` library, up to v0.5.0, introduced a bug that, when using
the internal watcher server and accessing a URL path without a trailing slash, would redirect
the user to an incorrect URL (by incorrectly injecting additional path segments). This version
of Statik fixes this by ensuring that at least v0.5.1 of `httpwatcher` is installed.
* Full refactor of Statik's view system. This was to make the view processing system more
modular and easier to debug.
* Regression test for 51. Due to the fact that this could actually be a common use case for
Statik, a regression test was created to ensure that the case brought up in 51 was possible
to implement using Statik.

* Fixed 47 - now allows for output of general non-HTML content,
depending on view path. Also related to 48.
* Added unit tests to verify 48 definitely works.