Renamed (header.html) -> (navigation.html)

Original name to ambiguous.

Rename:
(_source/_includes/header.html) -> (_source/_includes/navigation.html)
Update (gulpfile.js) with autoprefixer options.

Overwrite the default autoprefixer options in stylus task module
(./gulp/). Default did not account for CSS flex box usage.
Add (email.html) include for Jekyll.

Obfuscate my work email as a JavaScript script. Creating script as
Jekyll include will allow use as variable, maintaining modularity.
Rename (navigation.html) -> (header.html)

Original naming scheme was more semantically correct.