New typography scheme and improved readability

[gavinwray]

I've updated the theme's style in the following areas:

1. New typography scheme to improve readability on large and small screens.
2. A maximum width for the post container to provide a better line length and overall readability on large screens.
3. Revisions to the post container margins and padding to improve readability on small-screen devices.
4. Minor improvements to list styles, tables, blockquotes and preformatted code.
5. Increased the number of characters at which post titles are truncated in post fragments.
6. Display post date on post fragments and at the top of posts.

You can see the changes implemented on http://twangcity.postach.io