Remove bootstrap

---->>>>> **NOT READY FOR MERGE** <<<<<----

Done:
- Moved to newer standard for heading tags (h1 = title of section, not title of page)
- Removed all Bootstrap dependencies from main page HTML
- 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)
- Transferred all main page code from 18f.css to _custom.scss and condensed into Sass. There are a few remaining CSS issues to be resolved, but they are not substantial and do not relate to Bootstrap.
- 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~~ deleted bootstrap
- 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)~~ Cleaned it up.
- 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
- Got the team bios in order, including pure css solution to grayscale images to address #140 . 

Outstanding:
- Finish 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)
- Still need to check cross-browser support.
- Finish moving everything to HTML5 semantic div names (section, etc)
- Finish adding schema.org metadata to HTML tags in templates

Where I'm at:
- Basic Bourbon is covering the layout for the main page now.
- Bootstrap is removed.
- Next up: responsive code.

In general what I'm working towards:
- [ ] 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)