Various improvements and changes to typography

* Remove DroidSerif font in favor of Georgia
* Decrease body line height
* Decrease paragraph bottom margin
* Use black font color for content text
* Decrease .subtitle class font size and letter spacing
* Decrease .format-aside and .format-quote line height
* Decrease H2 font size
* Add 40px top margin and 15px bottom margin to entry content H1-H6 tags
* Decrease H1 entry content font size
* Use smaller font size for .subtitle class when browser < 500px
Use <button> tags on Next and Previous links to improve UX
Merge branch 'master' of github.com:raamdev/independent-publisher