refactored plugin loading into load_plugins.php
plugin now reads its package.json
base plugin for visualizations
singleton class to handle visualization registration
refactored pie-chart into a plugin
load visualizations from DatawrapperVisualization
bugfixes
refactored to use DatawrapperVisualization
Merge branch 'plugins' of github.com:datawrapper/datawrapper into plugins

Conflicts:
	lib/bootstrap.php
added some comments