Updated delete icon to Material Design

Copied from the official Google Material Design icon repo
Add momentary highlighting to Plane's language menu.
Move Code app to demos directory.
Remove core-unsupported languages from Code demo.  Fix resize in Graph.
Fix redirect on Code demo.
Fix illegal connections in stacks after a healing delete (issue 15).
Merge pull request #11 from JordiOrlando/master

Update delete icon to Material Design

Google has always been suspicious of the old trashcan artwork.  It came from MIT's Open Blocks, which despite claiming to be open source is known to have some copyrighted resources (click sound is stolen from MS Windows 95).  A thorough search could not locate any known origin for the trashcan, so we used it, warily.
Fade out trashcan when closed.
Add separators to toolbox.
Add XML unit tests.
Add red 'X' to mouse cursor if blocks are over the trash can.
Add red 'X' to mouse cursor if blocks are over flyout.
Convert Blockly.Toolbox from a singleton to an instance.
Fix coordinate bug introduced yesterday.
Use existing path constant for notch highlighting (issue #18).
Add red 'X' to mouse cursor if blocks are over toolbox.
Remove unneeded HTML attributes (Google style guide).
Fix stylesheet lookup to get correct sheet.
Use possessive 'its' instead of contraction for 'it is'.
Merge pull request #22 from BrianGeppert/master

Use possessive 'its' instead of contraction for 'it is'.