Added eos-help-center CSS, images, and fonts

[endlessm/eos-sdk#2231]
Put Endless customizations in separate file

We will keep all modifications to the XSLT stylesheets in our own file
as much as possible.

(Roberto, Will, Fernando, Philip)

[endlessm/eos-sdk#2231]
Add divs around content

Override the html.page transform in order to provide the expected divs
around the rendered Mallard document.

(Roberto, Patrick, Will, Fernando, Philip)

[endlessm/eos-sdk#2231]
Updated .gitignore

Added other exceptions to .gitignore
Refined CSS styling

Adjusted CSS files to improve layout.
These changes were required to make the eos-help-center design play nicely with Yelp.

[endlessm/eos-sdk#2232]
Improved CSS formatting

Fixed some CSS code-styling errors. This is by no means a complete review of style and standard compliance.

[endlessm/eos-sdk#2232]