Improved general readability of type (fixes issues #69, #71, #72)

**Status:**
Good to merge.

**Changelist:**
* Increase ```font-size``` on ```p``` elements in log pages
* Increase ```line-height``` on ```p``` elements in log pages
* Increase ```margin``` on ```h*``` elements in log pages
* Decrease ```font-size``` on hero headline on homepage for better readability on displays that are 1300px wide but less than 960px tall.
* Make images in log posts horizontally centred within the column