Merge branch 'alex_dev' into develop
Implemented functions `on` and `off` for Timeline events
Implemented item selection and a function select to get/set the selected items
`select` event now returns the original type of the id, not the stringfied ids
Splitted function `select` into `getSelection` and `setSelection`
Merge branch 'alex_dev' into develop
Renamed UI* stuff to Navigation
Renamed keyboardNavigation to keyboard
Moved images to src/graph/img, adjusted build script to copy them to dist, and some more variable renaming in the code
Tweaked the script to get icon path
Zoom via navigation now zooms around the window center
Added images to dist
Implemented functions `on` and `off` to create event listeners for event `select`.
Fixed non working `start` and `end` options.
Released version 0.4.0
Updated version number to 0.5.0-SNAPSHOT
Merge branch 'develop' into alex_dev

Conflicts:
	dist/vis.js
	examples/graph/02_random_nodes.html
	examples/graph/18_fully_random_nodes_clustering.html
	examples/graph/19_scale_free_graph_clustering.html
	examples/graph/20_UI_example.html
	src/graph/Graph.js
	src/graph/SelectionMixin.js