rerouting on 404, added 404 template

[evantarrh]

Closes issue #130.

[natebrennand]

I like this a lot. We should add a link to the documentation page, since they're usually going to hit a 404 when they're using a malformed URL

[natebrennand]

also, should it be "page", not "file"?

[evantarrh]

Cool, thanks––I made those changes and resolved conflicts with other PRs. The build still isn't passing, though––why is that?

[natebrennand]

here's the CI output:

$ flake8 density/
density/density.py:92:1: E302 expected 2 blank lines, found 1
The command "flake8 density/" exited with 1.

---

By the PEP8 standard, there must be 2 blanks lines between function declarations. You can install flake8 with pip and test files locally before pushing. (or run the super convenient tests.sh script)

[natebrennand]

It's possible to integrate flake8 or another pep8 syntax checker into your editor usually.

[jzf2101]

Looks cool! :+1: after Travis says it's ready to go

[evantarrh]

Thanks Nate! everything should be :ok_hand: now

[natebrennand]

:100: