Merge style guide bits from unit-e-docs repo into standard place in developer notes

[cornelius]

Merge https://github.com/dtr-org/unit-e-docs/blob/master/code-guides/style_guide.md into https://github.com/dtr-org/unit-e/blob/master/doc/developer-notes.md so that there is only one place where we define our coding style.

[scravy]

Fixed in dtr-org/unit-e#175

[cornelius]

@scravy I suppose we can delete the style_guide.md file from the docs repository then, right?

[scravy]

Yes, absolutely.

[cornelius]

Remove file: #46