Update layout to use YAML front matter variables.

- Add header key/value to YAML front matter block.
- Display page.title and page.header variables in layout template.
Add about me page.
Display posts on front page.

- Check if any posts exist.
- Loop over posts and display title.
Add MIT License for repo.

- Add disclaimer that content under _posts/ does not fall under MIT license.
- Update readme to include information about license.