Use `html2canvas` to assist thumbnail generation.

[xinan]

• Use html2canvas and use the old thumbnail functions to do cropping and resizing.
  • Able to capture html output (except cross-origin images)
  • Able to capture multiple turtles.
  • Able to capture turtle skins.

Note: I reverted the camera button because it turns out to be more complicated than I thought. There are several severe bugs that needs to be solved.

[xinan]

I wonder why one of the Travis threads fails.

[xinan]

@davidbau Could we sort out the test problem and get this merged first? Let's skip the camera button for this PR. I'm having another branch for that.

[davidbau]

OK. (The tests are still failing for me after we get rid of v10 etc.)

[xinan]

:tada: It passed after re-running.

[davidbau]

In this PR, what version of html2canvas is brought in? Since we're not using npm to install it, did you add it to bower.json so that grunt update updates it?

[davidbau]

This is very nice work. I like how the test is solid green now. I left a couple other comments.

[xinan]

Okay updated with response to all the suggestions above :smile:

• Fixed intermittent test error.
• Add html2canvas into bower.json and grunt bowercopy.
• Updated browser-side to not to generate blank thumbnails.
• Updated server-side to remove the existing thumbnail if a save request doesn't come with a thumbnail.
• Use an array to remember all the old style.display settings of the nodes that we are going to hide, and restore them when we are done capturing thumbnails.
• Made it not to zoom in when the thumbnail is too small, so we will not get a blur/pixelized thumbnail.

[davidbau]

Looks great. Merging!