Remove bootstrap

---->>>>> **NOT READY FOR MERGE** <<<<<----

Notes on progress:
- starting moving everything to HTML5 semantic div names (section, etc)
- started moving to newer standard for heading tags (h1 = title of section, not title of page)
- starting removing all Bootstrap dependencies
- transferred slideshow from JS to pure CSS solution (outstanding: remaining issue with links in captions — can only click on ‘first’ caption, no matter which z-index is placing which image/caption pair on top)
- started transferring all code from 18f.css to _custom.scss and condensing into Sass
- started transferring media queries into Bourbon-style media queries (outstanding: the queries are there in the parts of the page that I’ve gotten to so far, but they could be abstracted another layer to add the breakpoints themselves as variables so if we want to change them it is relatively easy to do)(ie, instead of writing ``max-width 550px`` over and over, we can have a variable named ``$mobile`` and be done with it)
- changed file structure to reflect Bourbon/Sass preferences, including adding ‘directions’ to the Sass compile folder in _config.yml so that Jekyll can watch and compile Sass on changes
- cleaned up ``<head>`` include to add section headings, and commented out bootstrap for the time being
- added normalize.css to start from a clean styling baseline
- added ‘official gov website’ banner across top, as this seems like a standard we’re working towards. It needs more styling (it is ugly right now)
- cropped slideshow images so that they are the same proportions going in, rather than cutting them to the correct side in-browser to make them easier to work with
- started adding schema.org metadata to HTML tags in templates

Where I'm at:
- about halfway down the page ;) (you'll see it -- it's where everything stops looking nice, approximately at the team photos)

In general what I'm working towards:
- transferring current site to Bourbon
- creating a layout for subpages like the dashboard, etc (and pull our current work on the dashboard into the site)
- creating a layout for basic, simple pages (for things like our open source policy, linking policy etc)
- creating a layout for subpages of the current content, such as the team, about 18F, etc (this may end up being the same as the dashboard layout in terms of Jekyll templates)(content may vary widely)