Fix #596: Switch docs to rtd theme

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