Fix #596: Switch docs to rtd theme

Fix #596 

Completed:
 -  [x] Added "sphinx_rtd_theme" to dev_requirements.txt
 -  [x] Removed old styling from ./_template/layout.html
 -  [x] Configured sphinx_rtd_theme in conf.py

To-Do:
 - [ ] Display freeseer logo near next to the "Freeseer" title 
 - [x] Use custom footer information (#621)