Fix #111 (clear block erroring out)

This was due to a missing extern for JSNetworkX.
Add conversion between digraphs and graphs

Closes #100.
Split nodes+edges into nodes and edges tabs

Improves readability. Fixes #102.
Add DOT parser

Closes #108.
Stringify network exception before passing it to showMessage()
Improve export to HTML
Fix examples broken due to splitting of nodes+edges
Remove redundant WordNet loader tool block

This has largely been supplanted by the new subgraph functionality.
Fix BFS and DFS blocks to have the correct category set
Make Edgy tool blocks available under Libraries

Closes #118.
Fix #119 (custom blocks not showing exception messages)

Also now shows an image of the failing block.
Refactor HTML export
Add export to ZIP

Thanks to @Craxic for his implementation, on which this is based on.
Fix last.fm friend getter erroring due to API idiosyncrasies

last.fm's crazy API returns undefined for no friends, a single friend
datum for a single friend, and an array for more than one.
Fix last.fm loved track getter erroring due to API idiosyncrasies
Merge pull request #124 from Muon/master

Bugfixes, export to ZIP, split nodes+edges, conversion between digraphs and graphs